Which organism was used by T.H. Morgan for his experiments?

The correct answer is Option (4) → Drosophila

Option (4) Drosophila — Correct. Of the options, only Drosophila (the fruit fly) is Morgan’s organism. Pisum (pea) was Mendel’s plant, Bombyx is the silkworm and Musca the housefly.

NCERT Extract:
Chromosomal Theory of Inheritance – “… Morgan worked with the tiny fruit flies, Drosophila melanogaster (Figure 4.10), which were found very suitable for such studies. They could be grown on simple synthetic medium in the laboratory. They complete their life cycle in about two weeks, and a single mating could produce a large number of progeny flies. Also, there was a clear differentiation of the sexes – the male and female flies are easily distinguishable. Also, it has many types of hereditary variations that can be seen with low power microscopes.”