A. Kingdom Animalia includes the multicellular organisms having eukaryotic cells with the heterotrophic mode of nutrition.
B. Kingdom Protista includes unicellular or colonial unicellular eukaryotic organisms which are mostly aquatic while the mode of nutrition can be autotrophic or heterotrophic.
C. Kingdom Fungi includes the multicellular eukaryotic heterotrophic organisms which are generally parasites and saprophytes.
D. Kingdom Plantae includes multicellular and photoautotrophic organisms which grow indefinitely.
Since decomposers are saprophytes, as they feed on dead and decaying matter, so they belong to Kingdom fungi.
So, the correct answer is 'Kingdom Fungi'.