Step 1: Recall the locomotory structures used by protozoans.
Amoeboid protists move using cytoplasmic projections, while others rely on whip like or hair like organelles built around a fixed axoneme.
Step 2: Rule out flagellum, cilium and pellicle.
Flagella and cilia are permanent organelles with a stable internal structure, and the pellicle is a supportive body covering, not a locomotory projection at all.
Step 3: Confirm pseudopodium fits the description.
Pseudopodia form and disappear as the cytoplasm shifts between a gel and sol state, which is exactly the temporary, cytoplasm based movement the question describes, as seen in Amoeba.
Final answer: Option 3, pseudopodium.