The principle says an interest tied to a condition fails once that condition becomes impossible to meet. Here, B's payment depended on marrying C, and C was already dead when A made the promise. Let's go through each option on its own terms.
The only option that fits the stated principle without adding a reason the facts don't support is the first one: B's interest fails simply because the underlying condition is impossible, with no need to dress that up as immorality or illegality.
So the correct answer is B's interest fails.
This problem turns on what happens to a conditional interest when the underlying condition can never be performed. Rather than testing each option against the facts one by one, it helps to ask what kind of mistake each wrong option makes.
Two of the wrong options invent grounds (immorality, illegality) the facts never raise, and the third simply contradicts the principle outright. Only the first option states the failure on the exact ground the principle supplies: impossibility of the condition.
Therefore, the correct answer is B's interest fails.