This question asks you to find the answer choice which explains the apparent inconsistency in the passage. The first step is to identify the inconsistency:
The scientist talks about how both Earth and Venus have hot molten cores and that both planets MUST expel the excess heat in some way. Then she says that on earth this happens through volcanoes and fissures, but on Venus it doesn't. The inconsistency here is that Venus must expel heat in some way but it doesn't do it in the way Earth does. So, you're looking for an answer choice which explains how Venus is able to deal with the extra heat from the molten core.
Answer (B) is the only answer choice which provides an explanation for how the heat from Venus's molten core is expelled aka that its thin surface allows the heat to radiate into space. The other answers all may talk about heat or the make up of Venus, but do not actually provide an explanation for what happens to the heat that Venus must expel.