Step 1: Cap cells are a distinctive feature associated with the unusual cell division pattern of the green alga Oedogonium.
Step 2: During cell division in Oedogonium, a ring of wall material is laid down near the apex, and after division this leaves behind cap-like scars (caps) at the top of the cell, a feature unique to this genus among common algae.
Step 3: Fucus, Ectocarpus and Polysiphonia are brown or red algae that do not show this cap cell phenomenon, so option (A) is correct.