Answer: option B. 1/2
Justification:
In general form, as per the definition of probability:
# of positive events or outcomes
probability = -----------------------------------------------
# of possible events or outcomes
Applied to this case, it results that the probability of chosing a point on segment AG, given that the point is chosen at random on the segment AM, is:
size of the segment AG
Probability = ---------------------------------------
size of the segment AM
Size of the segment AG = 6
Size of the segment AM = 12
=> probability = 6/12 = 1/2