Let $\ast$ be the binary operation on $N$ given by $a \ast b =$ LCM of $a$ and $b$. $(v)$ Which elements of $N$ are invertible for the operation $\ast$ ?

Question

$\begin{array}{1 1} \text{Only 1} \\ \text{1 and 0} \\ \text{Any element in N} \\ \text{Only a and b} \end{array} $