Step 1: Check the Assertion using the basic bound on probability.
Since $0\le P(E)\le1$ and $P(E)=0.2p$, we need $0.2p\le1$, giving $p\le5$. So Assertion (A) is true.
Step 2: Check the Reason on its own.
$P(\overline{E})=1-P(E)$ is a standard, always-true formula for complementary events, so Reason (R) is true.
Step 3: Test if the Reason actually explains the Assertion.
Reading it as "$p$ cannot exceed 5 BECAUSE $P(\overline{E})=1-P(E)$" does not logically connect, since the bound on $p$ came only from $P(E)\le1$, not from the complement formula.
Step 4: Conclude.
Both statements are true, but (R) does not explain (A), matching option (B).
\[ \boxed{\text{Option (B)}} \]