Who put forward the "Theory of linkage"?

Why did Morgan choose the fruit fly for his experiments to prove the chromosomal theory of inheritance?

$A$: Crossing over is the exchange of genetic material between non-homologous chromosomes.
$R$: It produces new linkages.

The first attempt to demonstrate linkage in plants was made in $.....$.

Based on which of the following did Morgan observe characteristics similar to Mendel's analysis experiments in the fruit fly Drosophila?

Genes $A, B$,and $C$ are linked. Genes $A$ and $B$ are closer than $A$ and $C$. Find out the correct option for the given statements:
$I.$ $A$ might be before $B$ and $C$
$II.$ $B$ might be between $A$ and $C$
$III.$ $C$ might be between $A$ and $B$
$IV.$ More crossovers have occurred between $A$ and $C$ than $A$ and $B$.