Final answer:
Increasing the number of grants for scientific research is most likely to increase productivity growth by promoting innovation and technology development, essential factors for economic growth.
Step-by-step explanation:
A policy with a reasonable chance of increasing productivity growth is increasing the number of grants for scientific research. Economic growth is fostered through investment in physical capital, human capital, and technology.
The government can enhance this growth through initiatives such as direct funding of R&D, tax incentives, and fostering cooperative relationships between universities and the private sector. This promotes innovation and the development of new technologies, which can substantially contribute to productivity growth across various sectors of the economy.