The correct answer is B) outer wires are worn to half their original diameter. The reason this is the correct answer is that wire rope should be renewed when the outer wires have worn down to half their original diameter. This is a standard industry guideline specified in regulations like the Code of Federal Regulations (CFR) for the US Coast Guard. When the outer wires have worn down this much, the wire rope's strength and integrity have significantly deteriorated, posing a safety risk. The other answer choices are incorrect because expiration of the certification period (A), outer wire rust (C), or a dry inner core (D) are not the primary criteria for determining when wire rope needs to be replaced. While these factors may also indicate potential problems, the wear of the outer wires is the key metric used to assess the condition and safety of wire rope.