It is true that:
$\text{all edge weights distinct}\Rightarrow\text{unique MST}$
But the converse is not always true.
Consider a graph that is already a tree:

The graph has equal edge weights.
However, there is only one possible spanning tree because the graph itself is already a tree.
Therefore, its MST is unique despite repeated weights.
Hence:
$\text{unique MST}\not\Rightarrow\text{distinct edge weights}$
So the "if and only if" statement is false.
Answer : B