While the phrase "must be true" is in this question stem, it's not a Must Be True question, it's an Assumption question! If we take a closer look at the question stem, we'll see that it says "according to the hypothesis, for
a person to use counting imaginary sheep as an effective method of inducing sleep, which one of the following must be true
?" Simply put, we're looking for something that has to be true in order for the conclusion
(in this case that counting sheep would help this person fall asleep) to follow
. As Adam mentioned, the verbiage "must be true" in this question points out just that -- we are looking for a necessary assumption here!
Here's a link to a helpful blog post that further discusses Must Be True vs Assumption questions: https://blog.powerscore.com/lsat/bid-28 ... edfellows/
. The post talks about how assumptions are unstated premises that must be true for an argument to be true, as well as differences between the two question types! I highly recommend reviewing it!
I hope this helps