Auxospores and hormogonia are formed, respectively, by

Explanation

d. Until 1907, auxospore formation was regarded as asexual process but now it is considered as an act of sexual process. The auxospores may be autogamous, isogamous, anisogamous or oogamous. Their pattern of formation differs in pennate and centric diatoms. Formation of hormogonia is the common method of reproduction in Nostoc which are produced by accidental breaking of trichome into several pieces. It may also be formed by death and decay of ordinary intercalary cells. Soon, the hormogonium escapes from mucilage and grows into a new filament and then into a new colony. Auxospores are formed by several diatoms, and hormogonia are produced by a few cyanobacteria. a. Some diatoms and several cyanobacteria: This option is incorrect. Auxospores are formed by diatoms, but hormogonia are not associated with cyanobacteria. b. Some cyanobacteria and many diatoms: This option is also incorrect. Auxospores are typically formed by diatoms, while hormogonia are mainly a feature of cyanobacteria. c.Several cyanobacteria and several diatoms: This option is incorrect as well. While diatoms do produce auxospores, the term "hormogonia" is more commonly associated with cyanobacteria. Summary: Auxospores are primarily produced by diatoms, while hormogonia are a feature of a select group of cyanobacteria. Option 4 correctly identifies the organisms associated with the formation of these structures.