Who discovered the mutagenic effect of $X$-rays?

$...$ is a phenomenon by which genetic variation is achieved through changes in the base sequences within genes,which creates a new character or trait absent in the parental generation.

The chromosome shown in the diagram below is broken at the points which are indicated by the arrows and the genes between these points become inverted. The resulting order of the genes will be:

The first definite proof of the mutagenic action of $X$-rays was given by

Which of the following is a classic example of a point mutation?

For what did $H.J. Muller$ receive the Nobel Prize?