Which cross is used to determine the genotype of the $F_1$ generation?

Given below are two statements: one is labelled as Assertion $A$ and the other is labelled as Reason $R$.
Assertion $A$: In an experiment, Mendel observed that the $F1$ progeny plants are all tall and none are dwarf.
Reason $R$: Stem height is a contrasting trait, with tall being dominant and dwarf being recessive.
In the light of the above statements, choose the most appropriate answer from the options given below:

Which of the following is $NOT$ one of the seven pairs of contrasting traits studied by Mendel in pea plants?

Mendel obtained recessive character in $F_{2}$ by ...$A$... the ...$B$... plants. Here $A$ and $B$ refer to

Which plant is used as a model system in the study of plant genetics?

Which of the following is the unit of inheritance?