Which boundary prevents the passage of energy as heat?

Adiabatic boundaries are designed to prevent the transfer of energy as heat between a system and its surroundings. This is fundamentally important in thermodynamics, as it allows for the study of processes where no heat exchange occurs, such as in adiabatic expansions or compressions of gases.

In contrast, a diathermic boundary allows heat to flow freely in and out of the system, which means it is not suitable for scenarios where one wants to isolate heat transfer effects. Open and closed boundaries pertain to the exchange of matter; an open boundary permits the transfer of both energy and matter, while a closed boundary allows only energy transfer (but not matter). The defining characteristic of an adiabatic boundary is its complete insulation from heat transfer, making it the correct choice for the question.