API RP 571 under Atmospheric Corrosion notes:
"The highest corrosion rates are typically observed between 80°F and 120°F (27°C-49°C), particularly when relative humidity is high and surfaces are wet from condensation or rain."
"At higher temperatures, surface dryness generally reduces corrosion activity despite increased reaction kinetics." (Reference: API RP 571, Section 4.3.1.3 - Atmospheric Corrosion) Thus, among the listed temperatures, 175°F is closest to the upper end of the most active corrosion window
, making option A the most accurate answer.

API RP 571 provides detailed insights:
"Brittle fracture generally occurs at low temperatures, specifically below the ductile-to-brittle transition temperature as defined by the Charpy impact test."
"Most carbon and low-alloy steels are ductile above their Charpy transition temperature but may fracture suddenly without ductility below it." (Reference: API RP 571, Section 4.2.1.2 - Brittle Fracture) Hence, option C is correct and technically accurate based on the material behavior curve.

API RP 571, in its discussion on Wet H#S Damage, including HIC, SOHIC, and SSC, consistently identifies a shared mechanism:
"All wet H#S-related damage mechanisms involve the absorption of atomic hydrogen, formed at the steel surface by the corrosion reaction in the presence of H#S."
"The absorbed hydrogen may diffuse into the steel, accumulate at trap sites, and cause cracking or blistering." (Reference: API RP 571, Section 4.2.2.7 - Wet H#S Damage Mechanisms) Thus, hydrogen absorption and permeation is a unifying mechanism across all these damage modes.
Therefore, option D is correct.

As defined in API RP 571, under Hydrogen Blistering and HIC/SOHIC:
"SOHIC typically manifests as subsurface stepwise cracking, generally oriented perpendicular to the direction of stress."
"This mechanism initiates below the surface, typically in weld heat-affected zones, and requires metallographic or advanced ultrasonic methods to detect." By contrast, SSC, caustic cracking, and amine SCC are primarily surface-initiated, stress-related cracking mechanisms.
Thus, SOHIC is the subsurface mechanism in this list, making option D correct.

According to API RP 571 Section 5.2.1 (Fatigue - High Cycle and Low Cycle):
"Unsupported small-bore attachments like vents and drains are prone to high-cycle mechanical fatigue due to vibration. These attachments experience cyclic stress, especially near welds or bends."
* Thermal overload is unlikely without over-temperature data.
* SSC (Option C) requires a wet H2S environment, which is not indicated.
* Weld defects may contribute but are less likely the root cause without other signs.
Hence, mechanical fatigue (Option A) is most probable in this configuration.