Asexual reproduction in fungi primarily occurs by mitosis.
Explanation: Fungi reproduce asexually through mitotic cell division, which allows them to produce genetically identical offspring. In this process, a single fungal cell divides into two identical daughter cells without the need for sexual reproduction. This is commonly seen in many fungal species, where structures like spores are produced. Binary fission (option a) is mainly a method of reproduction in prokaryotes, such as bacteria, and not in fungi. Meiosis (option b) and nuclear fusion (option c) are processes associated with sexual reproduction and genetic variation, which are not typical for asexual reproduction in fungi.