Who put forward the "Theory of linkage"?

Who clearly proved and defined linkage?

Given below are two statements: one is labelled as Assertion $(A)$ and the other is labelled as Reason $(R)$.
Assertion $(A)$:
Mendel's law of independent assortment does not hold good for the genes that are located closely on the same chromosome.
Reason $(R)$:
Closely located genes assort independently.
In the light of the above statements,choose the correct answer from the options given below:

The greater the length of the chromosomes in an organism, the greater the genetic variation. This variation is obtained from which of the following processes?

Crossing over is advantageous because it brings about

The phenomenon of crossing over in diploid organisms is responsible for