Sexual reproduction has never been observed in which type of fungus

The answer is fungi imperfecti or imperfect fungi which are also known as Deuteromycota, are fungi which do not classify into the commonly recognized taxonomic classifications of fungi that are based on biological species thought or morphological features of sexual structures because their sexual form of reproduction has never been observed. Merely their asexual form of reproduction is recognized in which means that these fungi yield their spores asexually in the procedure named sporogenesis.