With which economist is the concept of "Stages of Economic Growth" associated?

Given below are two statements, one labeled as Assertion (A) and the other as Reason (R): Assertion (A): Rostow suggested that underdeveloped economies should invest 20% of national income in capital formation to accelerate economic development. Reason (R): According to Rostow, raising the rate of saving and capital formation determines the pace of economic development in underdeveloped countries. In light of the above statements, choose the correct answer from the options given below:

According to the Lucas Growth Model, what is the central driver of long-term economic growth?