Firstly, it is not allowed to post the actual LSAT question, please consider just referring to the question PT #/Q# like you already have.
Secondly, try thinking about in percentages. Locking yourself into a defined hypothetical number can get you into trouble and it did to me early on. And remember, most=more than 50% in LSAT world.
New cars RM has= 100% of new cars that they have Most new cars from RM= 51%+ bought by B residents. Rest? Who knows. RM sold more new cars than ever. So this years 51%+ > any other year no matter what percentage New cars purchased by B residents not from RM. --> Therefore, RM's 51% (actual total)< New cars no matter where those new cars came from. Then if you must put a number to it at this point, anything that is a lower number for RM than all new cars works. Maybe RM sold 4/10 new cars in the whole city to B Residents which mean 6/10 were from other dealers. Please note we don't care here about all the new cars that were NOT sold and are still sitting on the lot. We just need "most" of the other new cars that were sold to equal any actual number that is higher or equal to 51% of the total.
There are a lot of ways to think about answer choices that can be valid, but here is what goes through my head.
A. I dunno? Just says RM sold more than ever, not more than ever to B residents B. If RM sold more new cars than ever, this years 51%+ has to be larger than ever in quantity, so the amount of the rest of the sale of new cars has to be more quantity too. Which makes this tempting, however, just because RM sold more this year than ever, maybe they sold zero in all other years and maybe like 2 years ago people bought more total new cars? For example, 2 years ago residents bought 20 new cars from all dealerships total, but RM sold zero, and has always sold zero new cars. This year, people bought 15 new cars total and RM sold 2 out of 3 new cars to residents they had on the lot. Wow, highest sales for RM ever! But still doesn't help out answer choice B. C. Nah, we don't know if there was 1 dealer that sold all the rest of the new cars or 20. If RM sold 5 cars and 20 other dealers sold one new car each, then no this answer is out. D. Well we definitely know this has to be true to make all the rest of the new cars=most. This one is super solid. E. I dunno because what if like 100 new dealerships opened up last year and VW made 1,000 extra new cars to send to them? Then maybe RM's share stayed the same or even went down due to ratio differences.
Hope this helps! Even if B seems trappy and you don't have time to fully think about it, we can be confident that D is the choice because it is what the stimulus depends on.