The substance which is used to stimulate the increase in apple size is
Promalin is a mixture of two naturally occurring plant growth regulators. Gibberellic acid, which cause cell enlargement and elongation; and 6-benzyladenine which promotes cell division. Promalin has a number of effects in apples, depending on when it is applied. It improves fruit size and finish. It also helps in increasing the branching in nursery trees.
So, the correct answer is 'Promalin'