Hint: Mendel has chosen the pea plant for the hybridization experiment because in his garden he has seen many varieties of pea plant which obtain from a similar seed. These plants differ in shape, size, and colour of plant parts.
In 1865, Mendel had completed his hybridization experiment on the 30,000 pea plants. Mendel has chosen the Edible pea plant because pea plant produces a large variety of different types of seed, it does not require a complicated procedure for cultivation, it is bisexual, pollination occurs in bud condition and the plant produce a fertile hybrid.
Mendel started his work with 34 varieties then after breeding, he selected 22 and finally worked on 7 varieties. Mendel worked on 7 contrasting characters of pea plant like-stem height(Tall/dwarf), flower color(Violet/white), flower or pod position( Axial/terminal), pod Shape (Inflated/constricted), pod color(Green/yellow), seed shape(Round/wrinkled), Seed color(Yellow/green).
Mendel has carried out a hybridization experiment by emasculation, bagging, dusting, tagging, crosses, and collection of seed. After the experiment, Mendel got a 3:1 ratio in all characters.
Additional Information: After doing experiments on pea plant Mendel concluded that out of two dissimilar characters only dominant can express and recessive is unable to express. But Mendel’s conclusion failed in some plant where the recessive allele also expresses itself with the dominant allele and forms an intermediate character that does resemble any of the main characters of the plant. Such a plant shows incomplete dominance. Incomplete dominance was discovered by Correns in 1903 on Mirabilis Jalapa and Snapdragon.
So, the correct answer is 7.
Note: Mendel performed many experiments in his time but did not get that much of recognition due to many of the reasons, his statical calculation is also beyond the comprehension of many biologists, but after knowing about his pea experiment many scientists have reworked on it.