Hugo de Vries worked on which plant and proposed the idea of mutation?

The type of genetic mutation that involves the substitution of a purine with a pyrimidine or vice versa (or the replacement of one type of base with another type of base) is known as:

The point mutations that induce changes in $DNA$ as $A \to G$,$C \to T$,$C \to G$,and $T \to A$ are respectively:

What do micro-mutations or changes involve?

Select the incorrect option for mutation.

$A$: Sickle cell anaemia occurs due to a point mutation.
$R$: $mRNA$ produced from the $Hb^S$ gene has $GUG$ instead of $GAG$.