Final answer:
Actin fibers are involved in phagocytosis, where a cell engulfs a large particle. Pinocytosis and receptor-mediated endocytosis do not use actin fibers.
Step-by-step explanation:
Actin fibers are involved in phagocytosis, which is the process of a cell engulfing a large particle. In phagocytosis, the cell membrane surrounds the particle and pinches off to form an intracellular vacuole. Actin fibers help in the movement and contraction of the cell membrane during this process. On the other hand, pinocytosis and receptor-mediated endocytosis do not involve actin fibers. Pinocytosis is the process of a cell engulfing small particles in fluid, and receptor-mediated endocytosis is a selective process where the uptake of substances is targeted to specific receptors on the cell membrane.