Which of the following pair of contrasting traits was not studied by…

The correct answer is Option (1) → Pink and white flowers

Option (1) Pink and white flowers — Not a pair studied by Mendel. Mendel’s flower colour pair in the pea was violet and white. Pink flowers come up in the snapdragon, where a cross of red and white gives pink by incomplete dominance.

Option (2) Inflated and constricted pods — A pair studied by Mendel. These are the two forms of pod shape.

Option (3) Axial and terminal flowers — A pair studied by Mendel. These are the two forms of flower position.

Option (4) Green and yellow pods — A pair studied by Mendel. These are the two forms of pod colour.

NCERT Extract:
Mendel’s Laws of Inheritance – “… Mendel conducted such artificial pollination/cross pollination experiments using several true-breeding pea lines. A true-breeding line is one that, having undergone continuous self-pollination, shows the stable trait inheritance and expression for several generations. Mendel selected 14 true-breeding pea plant varieties, as pairs which were similar except for one character with contrasting traits. Some of the contrasting traits selected were smooth or wrinkled seeds, yellow or green seeds, inflated (full) or constricted green or yellow pods and tall or dwarf plants (Figure 4.1, Table 4.1).”