The beginning of this question states “…the client asks the architect for a rule of thumb to use in determining how many square feet the parking lot will be” and then goes on to ask “How many square feet per parking space should be allotted…”.
Since the beginning of the question notes that the number we’re looking for is going to be used in determining how many square feet the parking lot will be, it’s reasonable to assume that the factor used should include both the parking spaces and the drive aisles. We’re also talking about the schematic design phase and a ‘rule of thumb’ - this implies we’re looking for an estimate of the square footage to be used by each space, not the actual mathematical area of a parking space.
Another tip - two of the possible answers are simply the area of a 9x18 spot and a 10x20 spot, respectively. If the question was looking for the area of a space to be used, one of these two would be correct. However, the question/scenario give no indication as to which size space will be used on the project - it’s impossible to choose between these two choices. Whenever you find yourself in this type of a scenario on a multiple choice question, it’s fair to assume both of those answers (and the logic that lead to them) are not correct.