RequirePackage{lineno}
documentclass[aps,prl]{revtex4}

usepackage{graphicx}% Include figure files

usepackage{epstopdf}   % this graphic package converts eps to pdf for easier usage of eps files

usepackage{lpic}
usepackage{amsmath,amssymb}
usepackage{nicefrac}
% Useful stuff
usepackage[utf8]{inputenc}
usepackage{hyperref}
hypersetup{
    colorlinks=false,       % false: boxed links; true: colored links
    colorlinks=true,
    linkcolor=blue,          % color of internal links (change box color with linkbordercolor)
    citecolor=red,        % color of links to bibliography
    filecolor=magenta,      % color of file links
    urlcolor=cyan           % color of external links
}
usepackage{xcolor}
usepackage{listingsutf8}
usepackage[section]{placeins}
lstset{mathescape=true}
% some shortcuts for later

% extended bibliography item
% mybibitem{label}{authors}{title}{year}{rest}
%newcommand{mybibitem}[5]{
%bibitem{#1} textsc{#2}, {it #3} ({bf #4}). #5}

newcommand{ket}[1]{left|{#1}rightrangle}
newcommand{bra}[1]{leftlangle{#1}right|}
newcommand{is}[1]{{color{blue} #1}}


%renewcommand{vec}[1]{ensuremath{overrightarrow{#1}}}
%usepackage{bbold}

% eprint shortcut
%newcommand{eprint}[1]{
%texorpdfstring{href{#1}{eprint: #1}}{eprint: #1}
%}
usepackage{subcaption}
%usepackage{subfigures}
begin{document}
linenumbers
begin{center}
Large{textbf{Problem with Figures}}
Underneath you may find some
figures (figures ref{fig1:a},  ref{fig1:b},  ref{fig1:c},  ref{fig1:d},  ref{fig1:e}  ref{fig1:f})
and
figures (figures ref{fig2:a},  ref{fig2:b},  ref{fig2:c},  ref{fig2:d},  ref{fig2:e}  ref{fig2:f}).

begin{figure}[htp]
  % Maximum length
  subcaptionbox{1alabel{fig1:a}}{includegraphics[width=1.6in]{example-image-a}}hfill%
  subcaptionbox{1blabel{fig1:b}}{includegraphics[width=1.6in]{example-image-a}}%

  bigskip

  % Equal length
  hspace*{fill}%
  subcaptionbox{1clabel{fig1:c}}{includegraphics[width=1.6in]{example-image-b}}hfill%
  subcaptionbox{1dlabel{fig1:d}}{includegraphics[width=1.6in]{example-image-b}}%
  hspace*{fill}%

  bigskip

  % Fixed length
  centering
  subcaptionbox{1elabel{fig1:e}}{includegraphics[width=1.6in]{example-image-c}}hspace{1em}%
  subcaptionbox{1flabel{fig1:f}}{includegraphics[width=1.6in]{example-image-c}}
    caption{Sample figures I}
    label{fig:1}
end{figure}

and

begin{figure}[htp]
  % Maximum length
  subcaptionbox{2alabel{fig2:a}}{includegraphics[width=1.6in]{example-image-a}}hfill%
  subcaptionbox{2blabel{fig2:b}}{includegraphics[width=1.6in]{example-image-a}}%

  bigskip

  % Equal length
  hspace*{fill}%
  subcaptionbox{2clabel{fig2:c}}{includegraphics[width=1.6in]{example-image-b}}hfill%
  subcaptionbox{2dlabel{fig2:d}}{includegraphics[width=1.6in]{example-image-b}}%
  hspace*{fill}%

  bigskip

  % Fixed length
  centering
  subcaptionbox{2elabel{fig2:e}}{includegraphics[width=1.6in]{example-image-c}}hspace{1em}%
  subcaptionbox{2flabel{fig2:f}}{includegraphics[width=1.6in]{example-image-c}}
    caption{Sample figures II}
    label{fig:2}
end{figure}

end{document}