Step 1: Read both statements.
The assertion is that transition metals and their complexes show catalytic activity. The reason is that the activation energy of a reaction is lowered by the catalyst. We judge each and the link between them.
Step 2: Examine the assertion.
Transition metals and their compounds are well known catalysts, for example $Fe$ in the Haber process, $V_2O_5$ in the Contact process and $Ni$ in hydrogenation. So the assertion is correct.
Step 3: Explain why they catalyse.
They are good catalysts because of their variable oxidation states and their ability to form intermediate complexes, which open an easier reaction path.
Step 4: Examine the reason.
A catalyst works by providing an alternative pathway with a lower activation energy, so the reason is a correct general statement about catalysts.
Step 5: Check whether the reason explains the assertion.
The reason describes how any catalyst speeds up a reaction, but it does not say why transition metals in particular are good catalysts. The specific cause is variable oxidation states and complex formation, so the reason is true but not the correct explanation.
Step 6: Conclude.
Both statements are correct but the reason is not the correct explanation of the assertion. The answer is
\[ \boxed{\text{Both (A) and (R) correct, (R) not the correct explanation}} \]