The lower group of plants like algae exhibit great variation in mode of sexual and asexual reproduction. Some algae produce gametes which are not similar in shape, size and structure, when they fuse, it is called anisogamy. e.g., Chlamydomonas.
The other options are incorrect because oogamy is the fusion of big oospore female with small male gamete. Isogamy is fusion of similar gametes. Zoogamy is sexual reproduction of animals.

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