The sentence that most clearly shows internal conflict is B. Frank wants to buy a house, but he is afraid to take on a mortgage.
An internal conflict is a struggle which the mind of a character has to deal with. In other words, a character that has an internal conflict must solve an emotional problem. Sentence B. shows that Frank is struggling with his own mind. In this case, the internal conflict has arisen from doubt (whether to take on a mortgage or not), which, at the same time, causes fear. On the other hand, the other options do not present mental struggles. They present external conflicts, that is to say conflicts between a character and something or someone that is outside his/her mind.
