Why do you think that only single-celled organisms live in the hadalpelagic zone? give a detailed answer.

No. Not only single-celled organisms live in the hadalpelagic zone

Explanation:

The deepest zone of the sea is called  “The Trenches” or Hadalpelagic Zone. This zone is described as beginning at approximately 19,000 feet and extending to the ocean floor. At this depth, there is no visible light, so there are no plants, resulting in very little food being available to sustain animal life.

Without single-celled organisms, there are many more organisms in this zone. They are not single-celled. Some of them are:

These all are multi-cellular.

an organism's phenotype is the  form of a trait  that it  displays. for  flowercolor, a pea  plant  can  display  a phenotype of purple or white flowers. an organism's  genotype  is the alleles of a gene it contains. based on his data, mendel concluded that a phenotype can be  determined  by more than one  genotype.

not sure if this is what you'r looking for eubacteria and archaebacteria