API RP 571 explains that both Sulfide Stress Cracking (SSC) and SOHIC are:
"Strongly influenced by material hardness. Carbon and low alloy steels with hardness exceeding 22 HRC (248 Brinell) are most susceptible."
"They occur in hardened steels under tensile stress, in the presence of wet H#S environments." (Reference: API RP 571, Sections 4.2.2.1 and 4.2.2.7 - SSC and SOHIC) Therefore, hardened steels are the common factor for both mechanisms, making option C the correct choice.

API RP 571 on Brittle Fracture notes:
"The primary factor influencing brittle fracture is material toughness, especially at low temperatures."
"Materials with low fracture toughness are susceptible to catastrophic failure when stressed below their ductile-to-brittle transition temperature." Hence, option A is correct.

According to API RP 571, in the section on Boiler Water Condensate Corrosion:
"The major contributors to condensate corrosion are dissolved CO# and O#. Carbon dioxide forms carbonic acid in the presence of water, lowering pH and causing generalized corrosion."
"Oxygen causes pitting and localized corrosion unless chemically treated." Thus, option B (Carbon dioxide and oxygen) is the primary root cause and the correct answer.

According to API RP 571, under Sulfuric Acid Corrosion:
"The most aggressive corrosion typically occurs below 65% H#SO#, particularly between 20% to 50% concentration range."
"Above 95% concentration, sulfuric acid becomes less corrosive to carbon steel due to its dehydrating nature."
"Dilute sulfuric acid is highly ionized and conducts electricity well, increasing corrosion rates." (Reference: API RP 571, Section 4.3.3.2 - Sulfuric Acid Corrosion) Hence, the most severe corrosion occurs below 65% concentration, making option B the accurate answer.

API RP 571 on Thermal Fatigue:
"Mixing of hot and cold streams, particularly where hydrogen is present, can induce thermal cycling stresses, leading to thermal fatigue cracking, especially at fittings like tees."
"Hydrogen embrittlement typically affects stressed zones, but this pattern strongly indicates thermal fatigue." So the correct answer is C - Thermal fatigue.