how to label and refer subfigures under tabular environment?

Question

I have two subfigures and I need to refer them in text such as Fig. 1.1(a) and Fig. 1.1(a) using Fig. \ref{fig1.1a:chap1} and Fig. \ref{fig1.1b:chap1}. The output is not coming. Instead, a double question mark (Fig. ??) appears. How to resolve this?

\documentclass[12pt, a4paper, oneside]{Thesis}
\usepackage{amsmath}
\usepackage{graphicx}
\usepackage{caption}
\usepackage{subfigure}

\begin{document}
\begin{figure}
    \centering
    \begin{tabular}{@{}c@{}}
        \includegraphics[width=\linewidth,height=125pt]{enrollment} \label{fig1.1a:chap1} \\[\abovecaptionskip]
        \small (a) Enrollment phase 
    \end{tabular}
    \vspace{\floatsep}
    \begin{tabular}{@{}c@{}}
        \includegraphics[width=\linewidth,height=125pt]{verification} \label{fig1.1b:chap1} \\[\abovecaptionskip]
        \small (b) Verification phase 
    \end{tabular}
    \caption{Mode of operations in a biometric authentication system}\label{fig1.1:chap1}%
\end{figure}
\end{document}

Answer 1

your approach to show two sub figures is at least unusual, frankly said wrong:

• you load obsolete package subfigure but than not use it. loading it is not sufficient, you should use its environment in figure composition and than compile document at least twice
• package subfigure is deprecated. use subfig instead it or subcaption (which is more powerful).
• references work on labels, which follows captions. that is not a case in your code example
• it is not clear, what is purpose of tables used in your images

• see if the following example works for you (i didn't use thesis document class, since i don't know which one you use. but this not influence on obtained result). after two compilation you will obtain working references

  \documentclass{report} % <--- instead of "thesis" which i haven't
  \usepackage{amsmath}
  \usepackage{graphicx}
  \usepackage{caption}
  \usepackage{subcaption} % <--- it is not compatible with "subfigure"
                          % with it you had to replace "subfigure"
  
  \usepackage{lipsum}     % <--- for dummy text
  
  \begin{document}
  \lipsum[1]
  \begin{figure}[htb]     % <---
      \centering
      \begin{subfigure}[b]{0.6\linewidth}     % <---
          \includegraphics[width=\linewidth]{example-image-duck}
          \caption{Enrollment phase}          % <---
          \label{fig1.1a:chap1}
      \end{subfigure}                         % <---
  
      \begin{subfigure}[b]{0.6\linewidth}     % <---
          \includegraphics[width=\linewidth]{example-image-duck}
          \caption{Verification phase}        % <---
          \label{fig1.1b:chap1}
      \end{subfigure}                         % <---
      \caption{Mode of operations in a biometric authentication system}
      \label{fig1.1:chap1}%
  \end{figure}
  Test of references: see subfigures \ref{fig1.1a:chap1}  and \ref{fig1.1b:chap1} in figure \ref{fig1.1:chap1}.
  \end{document}
  

with % <--- are indicated changes in comparison to your document example. if you like to have wide images, than change width of subfigure to desired width.

addendum:

you may be lucky and will can use subfigure package ... :-) :

    \documentclass{report}
    \usepackage{amsmath}
    \usepackage{graphicx}
    \usepackage{caption}
    \usepackage{subfigure} % <---

    \usepackage{lipsum}

    \begin{document}
    \lipsum[1]
    \begin{figure}[htb]     % <---
        \centering
        \subfigure[Enrollment phase \label{fig1.1a:chap1}]%
        {
        \includegraphics[width=0.6\linewidth]{example-image-duck}
        } 

        \subfigure[Verification phase \label{fig1.1b:chap1}]%
        {
        \includegraphics[width=0.6\linewidth]{example-image-duck}
        }
        \caption{Mode of operations in a biometric authentication system}
        \label{fig1.1:chap1}%
    \end{figure}
    Test of references: see subfigures \ref{fig1.1a:chap1}  and \ref{fig1.1b:chap1} in figure \ref{fig1.1:chap1}.
    \end{document}

result is similar as before, however you may experienced new problems with it.