Answer
See below
Work Step by Step
Given $V=P_2(R)$
We can write set $S$ as $S=\{p \in P_2(R):p(x)=ax^2+1\}$.
We can notice that $0x^2+0x+0\ne 0$. Hence, $S$ is not a subspace of $P_2(R)$
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