Representing a sentence using propositional logic
The sentence says that a male "is smart (S) if he is tall (S)", that gives $T \Rightarrow S$, but it also says "but otherwise is not smart", which gives $\neg T \Rightarrow \neg S$, and this is equivalent to $S \Rightarrow T$ by contraposition. Thus together this gives $S \iff T$ under the condition $M$. Therefore (b) is correct.