The correct answer is Option (1) - anisogamous

The fusion of two gametes dissimilar in size, as in species of Eudorina, is termed as anisogamous. In anisogamous reproduction, the gametes involved differ in size, with one being larger (usually the female gamete) and the other smaller (usually the male gamete). This type of reproduction is common in various organisms, including certain algae like Eudorina.