palindromes

For a string w, we define w R to be the reverse of w. For example, if w = 01101 then w R = 10110. Which of the following languages is/are context-free?

$$S \to aSa\,\left| {\,bSb\,\left| {\,a\,\left| {\,b} \right.} \right.} \right.$$ The language generated by the above grammar over the alphabet $$\left\{ {a,\,b} \right\}$$ is the...