API RP 571 states under the section Phosphoric Acid Corrosion:
"Corrosion usually occurs when the acid dries out and forms concentrated phosphoric acid deposits, which are very aggressive to carbon steel."
"Drying or concentration occurs due to poor flow distribution or operational upsets. Localized areas may experience acid concentration due to vaporization." (Reference: API RP 571, Section 4.3.3.8 - Phosphoric Acid Corrosion) Therefore, the most aggressive corrosion occurs when the acid dries out, making option D correct.

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.

API RP 945, dedicated to environmental cracking in amine units, states:
"Carbon steel is widely used in amine systems but is particularly susceptible to localized corrosion, under- deposit corrosion, and stress corrosion cracking, especially in high-pressure or rich amine environments."
"Proper control of amine quality, oxygen ingress, and temperature is critical to avoid accelerated corrosion in carbon steel equipment." Thus, carbon steels are the most commonly affected materials, making option C correct.

API RP 571 explains the typical location of dissimilar metal weld (DMW) cracks as follows:
"DMW cracking usually occurs in the heat-affected zone (HAZ) on the ferritic side of the weld."
"The difference in thermal expansion coefficients, grain structures, and creep rates between the ferritic and austenitic materials contributes to the development of high stress and strain concentrations in the HAZ of the ferritic material." (Reference: API RP 571, Section 4.2.1.6 - Dissimilar Metal Weld Cracking) Thus, these cracks do not occur in the center of the weld or on the austenitic side, but rather at the ferritic HAZ, making option A correct.

API RP 571 under Polythionic Acid Stress Corrosion Cracking (PTA SCC) specifies:
"Cracking is usually intergranular, and often localized, especially in sensitized austenitic stainless steels exposed to sulfur-containing environments during shutdown or cleaning."
"It is not always visible on inspection and may only become evident when a leak occurs." (Reference: API RP 571, Section 4.2.2.5 - PTA SCC) Hence, option C is correct as it best describes the detection difficulty and behavior of PTA SCC.