= Solution
Buy one lower-strike <European call option> and sell one higher-strike <European call option>. The initial cost is $C(K_i)-C(K_{i+1})<0$, so the strategy releases strictly positive cash. Its terminal payoff is
$$
(S_1-K_i)^+-(S_1-K_{i+1})^+\geq0
$$
for every <stock> price. Thus it is an <arbitrage>: a positive initial receipt accompanies a nonnegative terminal obligation. One may consume the receipt immediately, or hold it in cash to make a zero-initial-capital strategy with strictly positive terminal wealth. This is the <vertical-spread arbitrage for increasing call prices>.
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