Which one of the following statements is NOT correct ?

The incorrect statement is Option B. While the vast majority of fungi are multicellular organisms, Yeast (such as Saccharomyces cerevisiae) is a well-known exception and is unicellular.

Analysis of other options:

• Option A: According to R.H. Whittaker's Five Kingdom classification, all single-celled eukaryotes are placed under Kingdom Protista.
• Option C: Fungi are indeed a unique kingdom of heterotrophic organisms. They do not perform photosynthesis and obtain nutrients through absorption.
• Option D: Ascomycetes, commonly known as sac-fungi, are mostly multicellular (e.g., Penicillium, Aspergillus), but the group also includes unicellular forms like yeasts. They are primarily saprophytic, decomposers, or parasitic.