API RP 571 describes temper embrittlement as a subtle metallurgical change that reduces impact toughness without obvious macrostructural change:
"Temper embrittlement is not readily apparent by visual or standard tensile testing. It is confirmed by Charpy impact testing which measures toughness at low temperatures."
"It typically affects Cr-Mo steels exposed to intermediate temperatures (650°F to 1100°F) for extended times." (Reference: API RP 571, Section 4.2.1.4 - Temper Embrittlement) Thus, the condition is not visually or physically apparent, but is confirmed through impact testing, making option D correct.