This doesn’t strengthen the argument, because Traintrack Inn can attract a steady flow of customers with average food. There’s no evidence that Traintrack Inn has improved its food to attract business.

This doesn’t strengthen the argument, because there’s no indication that Traintrack Inn increased its popularity by moving to its more convenient location. Also, we’re not trying to justify a claim that Traintrack Inn is more popular because of its location. We’re trying to justify a claim that we can expect Traintrack’s food to be ordinary because its location creates popularity.

(C) undermines the argument, because Traintrack Inn is more popular despite having worse food than Marva’s Diner.

Helps build a bridge from the premise to the conclusion. (D), restated, asserts that if a business does not need to improve its products to attract customers, then it will not improve its products. We know Traintrack Inn doesn’t need to improve its food to attract customers, because it’s in a convenient location. (D), then, allows us to conclude that Traintrack Inn will not improve its food. This is why it’s not surprising Traintrack Inn’s food is ordinary.

If there’s no relationship between quality of food and popularity, then we’d have no reason to think that Traintrack Inn’s popularity due to its location would have us expect its food to be ordinary. The author actually assumes that the popularity of a restaurant CAN impact the restaurant’s food. If the restaurant can be popular simply through its location, then its food will tend to be worse.