Which one of the following shows isogamy with non-flagellated gametes?

Isogamy involves the fusion of gametes which are morphologically similar but physiologically distinct. Such gametes are called isogametes and produced in vegetative cells called gametangia. Isogametes may be planogametes (e.g. Ulothrix, Chlamydomonas) or aplanogametes (e.g. Spirogyra). Some algae (Spirogyra, diatoms etc) produce non-flagellate gametes called aplanogametes; their fusion process is called aplanogamy or conjugation.