Requirements prioritisation and negotiation belongs to:

Explanation

Sommerville models requirements elicitation and analysis as a spiral of four activities — requirements discovery, requirements classification and organisation, requirements prioritisation and negotiation, and requirements specification — so prioritisation and negotiation sits squarely inside elicitation and analysis. Requirements validation is a later activity that checks the written requirements for consistency, completeness and realism. A feasibility study is the earlier, short study that decides whether the project is worth doing at all. Requirements reviews are one specific technique used within validation. Hence the answer is requirements elicitation.