It is illegal to park within:
Answer: C — both a and b
It is illegal to park within 15 feet of a fire hydrant and 50 feet of a railroad crossing. Therefore, the correct answer is both choices A and B. Parking within these distances can obstruct emergency services in the case of fires or impede the visibility of trains approaching the railroad crossing, causing potential accidents. So, option C is the correct choice as it includes both restrictions on parking near fire hydrants and railroad crossings.