Fruit thinning growth regulator is

The fruit thinning growth regulator is NAA.
NAA (naphthalene acetic acid) is a synthetic auxin plant growth regulator that will effectively promote return bloom when used at the thinning stage in trees that are biennial bearing. It is widely used as an effective thinner in apple orchards. When applied shortly after fruit set, some fruit abscise leading to improved fruit size and quality.