= Solution
Suppose an <arbitrage> existed. Its discounted terminal wealth would satisfy $V_T\geq0$ $\mathbb P$-almost surely and $\mathbb P(V_T>0)>0$. Equivalence of $\mathbb Q$ and $\mathbb P$ gives the same two statements under $\mathbb Q$, so
$$
\mathbb E_{\mathbb Q}V_T>0.
$$
But part (c), after the usual admissibility or localization needed for integrability, gives
$$
\mathbb E_{\mathbb Q}V_T=V_0=0,
$$
a contradiction. Hence \b[the existence of the stated martingale measure rules out arbitrage.]
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