R programming language symbol

It is just \textsf{R}, or even \textbf{\textsf{R}}

You can download the R logo from here and then include the following line in your preamble:

\newcommand{\Rlogo}{\protect\includegraphics[height=1.8ex,keepaspectratio]{Rlogo.png}}

Then you can just write wherever you want:

\Rlogo

And the result should look like the following:

For additional info: here.

Good luck!