Mendel, the father of genetics, conducted experiments on pea plants to study the inheritance of traits. In his experiments, Mendel selected pairs of true breeding pea plant varieties. True breeding plants are those that, when self-fertilized, produce offspring with the same traits as the parent.

Mendel selected pairs of pea plant varieties that were similar except in one character, with contrasting traits. This means that each pair of plants had different traits for one specific characteristic. By selecting pairs with contrasting traits, Mendel could easily observe and analyze the inheritance patterns of these traits.

The correct answer is option 'A', which states that Mendel selected 14 pairs of true breeding pea plant varieties. These pairs allowed Mendel to conduct controlled experiments and observe the inheritance of various traits.

Mendel's experiments involved observing traits such as flower color (purple vs. white), seed shape (round vs. wrinkled), seed color (yellow vs. green), pod color (yellow vs. green), flower position (axial vs. terminal), and stem length (long vs. short). For each of these traits, Mendel selected pairs of plants that were true breeding and had contrasting traits.

By carefully crossbreeding these pairs and analyzing the traits of the offspring, Mendel was able to determine the patterns of inheritance. He observed that certain traits were dominant over others, and that traits were inherited independently of each other.

Mendel's experiments with true breeding pea plant varieties laid the foundation for our understanding of genetics. His work provided evidence for the existence of discrete hereditary units (genes) and the principles of segregation and independent assortment. Mendel's discoveries revolutionized the field of genetics and paved the way for modern genetic research.

A true breeding line refers to the plant that has undergone continuous self-pollination and showed stable trait inheritance and expression for several generations.
Mendel (father of genetics) selected 14 true-breeding pea plant varieties, as pairs, which were similar except for one character with contrasting traits.
So, the correct answer is '14