Answer:
the fourth answer (<->)
Explanation:
"if and only if" (symbolized by "<->" or "iff") is a bidirectional logical relation between statements, where either both statements are true or both are false.
so, you only need to use 1:1 the definition you learned in class.
there is nothing else here.
the symbol of the 4th answer means exactly "if and only if", which is what we are looking for in the problem definition.
so, I am not sure what your problem of understanding this is. this is truly just a matter of definition.
p "if and only if" q
means that p is true only if q is true (there are no other cases to make p true, which might be the case for "p if q", meaning that when q is true then p is true, but p can be true also in other cases). and that means q is true only if p is true.