= Solution
The <involutions in an even dihedral group> are the half-turn $g^3$ and the six reflections $g^jh$. Thus the seven order-two <subgroups> are
$$
\boxed{\langle g^3\rangle,\qquad\langle g^jh\rangle\quad(j=0,1,2,3,4,5).}
$$
The half-turn is central, so \b[only $\langle g^3\rangle$ is normal]. Indeed $g(g^jh)g^{-1}=g^{j+2}h$, a different reflection; an order-two subgroup has only one nonidentity element, so each reflection subgroup fails the <normal subgroup> condition.
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