Step 1: Check the Assertion.
Manganese has the configuration $[\text{Ar}]3d^54s^2$, giving it 7 electrons available in the outer $3d$ and $4s$ levels. When all seven take part in bonding, manganese reaches $+7$, the highest oxidation state seen anywhere in the first transition series. So the Assertion is true.
Step 2: Check the Reason.
It is also a true general fact that transition metals show variable oxidation states, because both the $(n-1)d$ and $ns$ electrons lie close enough in energy to be used in bonding.
Step 3: Test whether the Reason explains the Assertion.
Saying transition metals show variable oxidation states is a broad statement that applies equally to iron, chromium or cobalt. It does not by itself explain why manganese in particular reaches all the way up to $+7$; that specific result comes from manganese having exactly five $3d$ and two $4s$ electrons all available for bonding.
Step 4: State the final match.
Both statements are true, but the Reason is too general to be the specific explanation for the Assertion.
\[ \boxed{\text{Both A and R true, R is not the correct explanation of A}} \]