Final answer:
Masaccio created the illusion of an extension of architectural space in his Holy Trinity fresco by painting a barrel vault using linear perspective.
Step-by-step explanation:
Masaccio created the illusion of an extension of architectural space in his Holy Trinity fresco by painting a barrel vault. A barrel vault is a type of architectural element that resembles a semicircular arch extended into a continuous curve. Masaccio used linear perspective to accurately depict the barrel-vaulted space, creating the illusion of depth and three-dimensionality in the painting.