Question

How sexual reproduction favours algae n fungi during unfavourable conditions?

Answer 1

Asexual reproductions make exact copies. So if one gets diseased , then it will die. If disease is contagious then everyone will die. But if they have reproduced by sexual reproductions , there will be certain variants that will somehow survive the disease. When favorable condition return they switch back to asexual reproduction.
Also Algae and fungi reproduce by spores so out of 100 or 1000 spores they produce sexually at least one will survive.