In algae, sexual reproduction takes place through the fusion of two

The correct answer is: Gametes

Sexual reproduction involves the formation of gametes and their fusion during the process called fertilisation. Depending upon the structure and behavior of gametes, there are different types of sexual reproduction. These are
(i) Isogamy Fusion of morphologically alike gametes which look and behave similarly is called isogamy
(ii) Anisogamy Fusion of morphologically dissimilar gametes, which may be motile or non-motile
(iii) Oogamy Fusion of a large non-motile egg or ovum with a smaller motile sperm (except in Rhodophyceae). The fusion of gametes is called oogamy