To solve this problem, we need to determine how many pairs of contrasting characters in pea plants were studied by Mendel in his experiments.$\newline$Let's analyze Mendel's experiments:$\newline \bullet$Mendel conducted hybridization experiments on pea plants (Pisum sativum) to study inheritance patterns.$\newline \bullet$He selected specific traits that exhibited clear contrasting characteristics.$\newline \bullet$Mendel studied a total of seven pairs of contrasting characters. These included:$\newline$ - Seed shape: Round vs. Wrinkled$\newline$ - Seed color: Yellow vs. Green$\newline$ - Flower color: Purple vs. White$\newline$ - Pod shape: Inflated vs. Constricted$\newline$ - Pod color: Green vs. Yellow$\newline$ - Flower position: Axial vs. Terminal$\newline$ - Plant height: Tall vs. Dwarf$\newline$Therefore, Mendel studied seven pairs of contrasting characters.$\newline$This corresponds to Option 4.