Partial melting of the mantle begins when the actual rock temperature at any depth intersects the line in Pressure-Temperature space called :

Asthenosphere

Explanation:

The asthenosphere is considered to be part of the upper mantle of the earth. It is characterized by its viscous nature, softness, and ductility.

It is found below the lithosphere and has a high temperature and pressure level that partially melts rocks turning it into a semi-molten state.

Hence, Partial melting of the mantle begins when the actual rock temperature at any depth intersects the line in Pressure-Temperature space called: ATHENOSPHERE

answer; (d) melting of sea ice;