The fruit fly Drosophila melanogaster was found to be very suitable for experimental verification of chromosomal theory of inheritance by Morgan and his colleagus because:
It reproduces parthenogenetically
A single mating produces two young flies
Smaller female is easily recognisable from larger male
It completes life cycle in about two weeks.
As we learn
Suitability of fruit flies for the experiments -
The fruit flies were very suitable for this experiment as they could be grown on simple synthetic medium, their life cycle is about two weeks, and single mating could produce large number of progeny flies
- wherein
Also, males and females were very clearly distinguishable. 
Correct option is 4