The only exception concerning sexual reproduction in fungi is that of:

Explanation

Option a) Conjugation fungi: Not the exception, as they undergo sexual reproduction by forming zygospores. Option b) Imperfect fungi: The exception, as they lack a known sexual reproductive stage. Option c) Sac fungi: Not the exception, as they undergo sexual reproduction by forming asci and ascospores. Option d) Club fungi: Not the exception, as they undergo sexual reproduction by forming basidia and basidiospores. Answer is "Imperfect fungi"Explanation: Perfect stage is sexual stage which is absent in Deuteromycota. That is why it is called imperfect fungi or fungi imperfecti.