Explanation:

- Option 1: Agro technology

Involves the development and improvement of farming technologies and practices. Not linked with BEL and ISRO collaboration.

- Option 2: GaAs solar cells

GaAs (Gallium Arsenide) solar cells are high-efficiency, light-sensitive materials used in solar power systems, especially in space technologies. BEL and ISRO are indeed collaborating on this technology.

- Option 3: Nuclear technology

Deals with the processes and use of nuclear energy and materials. Typically managed by nuclear-focused organizations, not BEL.

- Option 4: C-DAC

The Center for Development of Advanced Computing focuses on advanced computing and development, and is a separate entity from BEL-ISRO collaborations.