How many true-breeding pea plant varieties were selected by Mendel?

The correct answer is Option (3) → 14

Mendel worked with true-breeding pea lines. A true-breeding line has self-pollinated for many generations, so it shows the same trait every time. He studied seven pairs of contrasting traits, such as tall or dwarf plants and round or wrinkled seeds. Each pair needs two varieties, one for each trait. So he selected 7 × 2 = 14 true-breeding varieties.

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.”