Desulfurisation of hot metal is favoured by

Desulfurisation is the process of removing sulphur from hot metal (usually molten iron) to improve its quality.

This process is favoured by a reducing atmosphere because an oxidising environment can lead to reversion of sulphur back into the metal.

A basic slag (rich in lime, CaO) reacts with sulphur to form calcium sulphide (CaS), which is stable and can be easily removed.

Thus, a reducing and basic slag environment promotes maximum sulphur removal by encouraging the formation and retention of CaS in the slag phase.

Therefore, Option D is the correct answer as it provides the most favourable conditions for desulfurisation.