Crossing over causes

The coupling and repulsion theory was proposed by:

Complete linkage is observed in.....

Drosophila melanogaster has $8$ chromosomes in its somatic cell. How many linkage groups will it have?

Morgan hybridized yellow-bodied,white-eyed females to brown-bodied,red-eyed males and intercrossed their $F_{1}$ progeny. He observed that:
$(a)$ $F_{2}$ ratio deviated significantly from the $9 : 3 : 3 : 1$ ratio.
$(b)$ Both genes did not segregate independently of each other.
$(c)$ Recombinant types are not obtained in $F_{2}$ generation.
$(d)$ Both genes segregate independently of each other.
Select the correct set of statements:

Linkage in Drosophila was first discovered by