API RP 571 states under Chloride Stress Corrosion Cracking (Cl-SCC):
"Nickel contents above approximately 35% provide significant resistance to chloride stress corrosion cracking. Alloys such as Alloy 625 and Alloy 825 exhibit excellent resistance."
"Austenitic stainless steels with lower nickel contents, such as 304 and 316, are more susceptible to Cl-SCC." Hence, option C (35%) is correct.

API RP 571 on HTHA states:
"Acoustic emission testing is not currently a proven or reliable method for identifying HTHA in refinery components."
"HTHA typically occurs internally, not as surface blisters, and is not limited to welds. Austenitic (300 series) stainless steels are generally resistant at normal refinery conditions." (Reference: API RP 571, Section 4.2.1.3 - High Temperature Hydrogen Attack) Thus, option A is the most accurate statement.

API RP 571 and RP 939-C define sulfidation conditions as:
"Sulfidation of iron-based alloys generally becomes significant at temperatures above 500°F (260°C) in hydrocarbon streams with sulfur but without hydrogen."
"For hydrogen-containing environments, this threshold may be reduced to 450°F (230°C)." (Reference: API RP 571, Section 4.2.1.1 - Sulfidation; API RP 939-C, Section 3.1) Therefore, the correct answer is D.

API RP 571 under Oxidation states:
"Resistance to oxidation is improved significantly by increasing chromium content, as it promotes the formation of a protective chromium oxide (Cr#O#) layer."
"Nickel may aid in high-temperature strength but does not directly improve oxidation resistance like chromium does." (Reference: API RP 571, Section 5.1.1 - Oxidation) Hence, option A is correct.

API RP 751 (referenced in RP 571) lists materials appropriate for HF acid alkylation service. It specifies:
"Chrome-moly steels such as ASTM A-193 B5 (5% Cr) bolts are preferred for their resistance to HF acid- induced cracking."
"Standard low alloy steels (e.g., B7) are not considered adequately resistant." (Reference: API RP 571 summary & API RP 751, Section 5.6.3 - Bolting Materials) Therefore, the correct and industry-approved answer is option A.