This is a frequently-occurring question from students, but one that I love to answer (so thank you for asking!). Please rest assured that this is a Veritas-created question, and not a GMAT question at all. The lesson embedded within it demonstrates the danger of only including one definition of a term when there may be multiple. In this case "lemon" has two definitions - one is a citrus fruit, the other is a poorly-manufactured car (please note that, on the GMAT, you won't be responsible for slang definitions such as this).
This question is really designed within the arguments lesson to note the carryover between argumentative logic and the Data Sufficiency math questions, so let me draw a parallel argument using math to demonstrate:
All Lemons Are Yellow
My Car is a Lemon <----------------------> x^2 = 16
Therefore, My Car is Yellow <--------------------> What is the value of x?
In Data Sufficiency questions, you'll need to draw a conclusion that MUST BE TRUE to the question - in this case, you can only have one value of x. Because the statement "x^2 = 16" has two definitions (x = 4 and x = -4), you can't simply choose one to answer the question. Both definitions would need to guarantee the same answer in order to draw a conclusion. In this case, they do not, and so we can't conclude a specific value of x.
Please note that the mathematical version of this question is much, much more useful to you than the lemon example. When covered effectively in class, we can alert you to the use of argumentative logic on the math side of the exam, and that's the purpose. On test day, you won't see any lemon-car references, so forgive us for the 'folksy' example...it's really only useful to prove this point.