The correct answer is C) 4 B-II fire extinguishers must be in the machinery space of a 175-foot long fishing vessel propelled by engines with 4000 brake horsepower. According to the regulations in 46 CFR 34.50-10, vessels with engine spaces over 1,000 brake horsepower require one B-II fire extinguisher for every 1,000 brake horsepower or fraction thereof. Since the vessel in question has 4,000 brake horsepower, it requires 4 B-II fire extinguishers in the machinery space. The other answer choices are incorrect because 2 extinguishers would be insufficient, 3 would be too few, and 5 would exceed the regulatory requirement for this size vessel.