Step 1: Recall what makes an element a transition element.
An element earns the transition tag only when its atom, or the common ions it forms, has a d-subshell that is not completely filled. That is the whole test we need to apply here.
Step 2: Write out the configurations of Zn, Cd and Hg.
Zinc ($Z=30$) ends in $3d^{10}4s^2$ and loses its two $4s$ electrons to give $\text{Zn}^{2+}$ with $3d^{10}$. Cadmium ($Z=48$) ends in $4d^{10}5s^2$, giving $\text{Cd}^{2+}$ as $4d^{10}$. Mercury ($Z=80$) ends in $5d^{10}6s^2$, giving $\text{Hg}^{2+}$ as $5d^{10}$.
Step 3: Apply the definition to these numbers.
In every case above, both the neutral atom and its usual ion carry a completely filled d-subshell, never a partially filled one. So none of these three metals can be called transition elements, which is exactly what the Reason states, and this is the direct cause of the Assertion being true.
Step 4: Match with the given options.
Since both statements are correct and the Reason is the actual explanation of the Assertion, option (A) is the right choice. \[ \boxed{\text{Option (A): Both A and R are true, R is the correct explanation}} \]