The correct option is D Chlamydomonas
Chlamydomonas, which is an alga, produces flagellated gametes that are of the same size. Hence, they are flagellated isogametes.
Spirogyra also produces gametes of equal size. However, they are not flagellated.
Both Fucus and Volvox produce gametes that are of unequal sizes, hence they are oogamous.