Which one of the following organs of the United Nations Organisation performs legislative functions?

The General Assembly, in which every member State is represented, is the UN’s main deliberative organ: under Articles 10 to 17 of the UN Charter it discusses and makes recommendations, initiates studies for the progressive development and codification of international law, and approves the budget. Among the listed organs it is the one regarded as performing legislative functions, so the answer is "The General Assembly", although its resolutions are generally recommendations rather than binding laws. The Security Council’s role is the maintenance of peace and security, and the Trusteeship Council and ECOSOC have specialised functions.