The enumitem
package allows the simple specification of various list parameters, including (in this case) itemsep
to set the vertical gap between items.
\documentclass[11pt,a4paper]{article}
\usepackage[utf8]{inputenc}
\usepackage[margin=1in]{geometry}
\usepackage{tcolorbox,enumitem}
\newcommand\linegap[1]{\underline{\hspace{#1}}}
\begin{document}
\begin{tcolorbox}[width=10cm]
\begin{enumerate}[itemsep=1.5cm,leftmargin=0pt,itemindent=.5in]
\item
\linegap{35pt}:\linegap{35pt} \linegap{135pt}\\
\linegap{75pt} \hfill A
\item
...........:........... ..............................................\\
........................ \hfill A
\end{enumerate}
\end{tcolorbox}
\end{document}

I can think of various interpretations for what the OP means by the "B" issue. Here is a possibility. In addition to using itemsep
, the setspace
package is employed to temporarily set the line spacing:
\documentclass[11pt,a4paper]{article}
\usepackage[utf8]{inputenc}
\usepackage[margin=1in]{geometry}
\usepackage{tcolorbox,enumitem,setspace}
\newcommand\linegap[1]{\underline{\hspace{#1}}}
\begin{document}
\begin{tcolorbox}[width=10cm]
\begin{spacing}{2.5}
\begin{enumerate}[itemsep=1.5cm,leftmargin=0pt,itemindent=.5in]
\item
\linegap{35pt}:\linegap{35pt} \linegap{135pt}\\
\linegap{75pt} \hfill A
\item
...........:........... ..............................................\\
........................ \hfill A
\end{enumerate}
\end{spacing}
\end{tcolorbox}
\end{document}
