The correct option is A Chlamydomonas
Sexual reproduction takes place through fusion of two gametes. These gametes can be flagellated and similar in size (e.g chlamydomonas) or non-flagellated but similar in size (e.g spirogyra). Such reproduction is called isogamous.
Fusion of two gametes dissimilar in size, as in species of eudorina is termed as anisogamous.
Fusion between one large, non-motile female gamete and a smaller, motile male gamete is termed oogamous.
e.g. volvox, fucus