\documentclass[12pt, a4paper]{article}
\usepackage{graphicx}
\graphicspath{{./images/}}
\usepackage{fontspec}
\setmainfont{Times New Roman}
\usepackage{subcaption}
\captionsetup{textfont={it, normalsize}}
\captionsetup{skip=1pt}
\renewcommand{\thesubfigure}{\roman{subfigure}}
\begin{document}
The first sub-figure, Fig.~\ref{fig:dog-01}, contains an image of a dog, while the second sub-figure, Fig.~\ref{fig:dog-02}, contains image of yet another dog.
\begin{figure}[htbp]
\centering
\begin{subfigure}[b]{.48\linewidth}