The characteristic of a geometric progression is that the ratio of a term and its consecutive term remains the same. Thus, if he can show that f(n)/f(n-1) remains constant, he can prove f(x) to be a geometric progression. The third option is correct.
The best and most correct answer among the choices provided by your question is the third choice or letter C.
Jake just proved that "<span>that f(n) … f(n - 1) was a constant ratio."</span>
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