India's wheat yield revolution in the 1960s was possible primarily due to :-

The Green Revolution in India in the 1960s was based on high-yielding, semi-dwarf wheat varieties developed from Mexican wheat (Norman Borlaug), such as Sonalika and Kalyan Sona.
These varieties carried dwarfing genes (Norin-10 genes, which arose by mutation). Shorter, stiffer stems prevented lodging (falling over) under heavy fertiliser and irrigation, and allowed more of the plant's resources to go to grain.
Yield, plant height and similar characters are quantitative (polygenic) traits. The genetic changes that raised yield in these varieties were therefore changes in quantitative traits, which is the reasoning behind the answer key.
Wheat is a self-pollinated crop and the Green Revolution varieties were not $F_1$ hybrid seeds; chlorophyll content was not the basis of the increase.
So, as per the answer key, the wheat yield revolution was possible primarily due to quantitative trait mutations (the semi-dwarf, height-reducing mutations being one such change).