5 : 35 :: 8 : ? Select the number that completes the analogy.

The number is multiplied by the number two places above it, that is $n\times (n+2)$. Here $5\times 7=35$, so $8\times 10=80$. Each distractor uses a neighbouring multiplier and collapses on the first pair: 72 is $8\times 9$, but $5\times 6=30$ and not 35; 88 is $8\times 11$, while $5\times 8=40$; and 64 is merely 8 squared, against 5 squared being 25.