All the options are wrong. B, C and D can be easily ruled out, so let's focus on option (a) now.
As far as option (a) is concerned, it turns out that, even (a) is not the correct answer. How? It is because the grammar cannot generate all the strings having equal number of $a$'s and $b$'s. For example, $abba$ is one such string which cannot be generated by the above grammar. Then what is the best way to describe the above grammar? It generates strings having equal number of a's and b's provided that, the string starts and ends with different symbols i.e. Starting with $a$ and ending with $b$, or starting with $b$ and ending with $a$. So the conclusion is, all options are wrong.
Note: Even though A is wrong, in an exam scenario it will be the most appropriate option to mark as correct. So from an exam point of view, A may be marked in the OMR sheet, but from a conceptual standpoint, A is wrong.