The total amount of rock collected 31.2 kg. This means if how much rock Ari collected on the second expedition is x, then the equation 13.9 + x = 31.2 represents the situation. We can then solve for x to find how much rock he collected on his second expedition.

Subtract 13.9 from both sides.

This means he collected 17.3kg of rock on his second expedition, meaning the answer is C.