Be sure to first pick out the flawed pattern of reasoning in the stimulus.
Flaw: A statement must be true (the thieves gained access below ground level), because someone (the guard) says it's true. There is no evidence to back up why the statement must be true, and presenting a conclusion without providing a basis for that conclusion is flawed reasoning.
The key word here is must. A word like "must", although it might not seem all that significant, truly make the difference is these kinds of questions. Other choices might be very similar, but the correct answer choice could be determined based on one word. So read really carefully, and develop the skill of reading each word independently while also understanding the context of the entire passage ( a skill which doesn't necessarily come naturally, and isn't necessarily taught throughout school. Doing so will help you to improve on the LSAT.
Once you've found the flaw, match it up with the same flaw in one of the answer choices.
In choice B, a statement must be true (customers buy shirts at less than store's cost) because someone (store's competitor) says it's true. A conclusion is presented without an adequate basis for stating that conclusion.
It's a perfect match, and the answer choice presents a parallel flaw to the flaw in the stimulus.