An important feature of the third five-year plan was.

The correct option is Setting up examination research units in some universities.

During the Third Five-Year Plan (1961–1966) in India, the government placed a strong emphasis on education as a key driver of economic and social development. Alongside expanding access to education, there was a growing need to improve the quality of higher education and reform the evaluation standards.

To address this, the plan introduced qualitative measures for examination reform. A major feature of this plan was the decision to set up examination research and reform units in various universities. These units were established to:
- Conduct scientific research on student assessment and evaluation techniques.
- Develop methods to reduce subjectivity and rote memorization in exams.
- Improve the overall reliability, validity, and efficiency of the higher education examination system.

Other options, such as reducing the number of colleges or abolishing education commissions, run counter to the educational growth and reforms targeted during the five-year plans, making the establishment of research units the correct choice.