The correct option is (b) gibberellic acid.
Explanation:
The most important effect of GA is the stem elongation, in which GA induces internodal elongation or sub-apical elongation.
It has been confimed on several plants such as pea, bean, tomato, cabbage, etc. where a significant elongation of internodes is reported. Genetically dwarf plants like pea and maize show normal size in the presence of gibberellins.