The correct option is A Zygospore
In Ulothrix, two isogametes from two different filaments come together and fuse by their anterior end to form zygospore. The zygote germinates by meiosis and produces four haploid zoospores, each of which swims for a certain period, then loses its flagella and attaches to a substrate and grows into a new filament. Thus, the correct answer is option B.