JEE Math Practice Question

For a suitably chosen real constant a, let a function, $$f:R - \left\{ { - a} \right\} \to R$$ be defined by $$f(x) = {{a - x} \over {a + x}}$$. Further suppose that for any real number $$x \ne - a$$ and $$f(x) \ne - a$$, (fof)(x) = x. Then $$f\left( { - {1 \over 2}} \right)$$ is equal to :

1. A.$$ {1 \over 3}$$
2. B.–3
3. C.$$ - {1 \over 3}$$
4. D.3