Which of the following is not specifically allowed for sealing around a pipe that passes through an exterior wall?

Flashing is typically used in construction to direct water away from critical joints or openings in structures, particularly around roofs, chimneys, and where walls meet roofs. It is designed to prevent water intrusion and is less about sealing conduit passages, like pipes through exterior walls, than it is about providing moisture control. In contrast, caulk, foam sealant, and gasketing systems are explicitly designed to create tight seals around penetrations, providing the necessary barrier against air and moisture infiltration specific to plumbing applications.

Caulk is often used in small gaps and is effective at sealing around pipes where they pass through walls. Foam sealants can expand to fill larger gaps and also provide an effective moisture barrier. A gasketing system, which includes pre-formed seals, is engineered to fit around pipes to ensure a snug fit and an airtight seal. Since flashing serves a different function and is not categorized as a sealing method for pipes penetrating walls, it is the correct answer in this context.