Answer: Rule 139
Except as provided in rule 147, the term of office of members of Councils shall begin on 1 January following their election by the General Assembly and shall end on 31 December following the election of their successors.
By-elections
Rule 140
Should a member cease to belong to a Council before its term of office expires, a by-election shall be held separately at the next session of the General Assembly to elect a member for the unexpired term.
Secretary-General
Appointment of the Secretary-General
Rule 141
When the Security Council has submitted its recommendation on the appointment of the Secretary-General, the General Assembly shall consider the recommendation and vote upon it by secret ballot in private meeting.
Security Council
Annual elections
Rule 142
[based directly on a provision of the Charter (Art. 23, para. 2, as amended
under General Assembly resolution 1991 A (XVIII)). See introduction para. 23]
The General Assembly shall each year, in the course of its regular session, elect five non-permanent members of the Security Council for a term of two years [1].
Qualifications for membership
Rule 143
[based directly on a provision of the Charter (Art. 23, para. 1)]
In the election of non-permanent members of the Security Council, due regard shall, in accordance with Article 23, paragraph 1, of the Charter, be specially paid, in the first instance, to the contribution of Members of the United Nations to the maintenance of international peace and security and to the other purposes of the Organization, and also to equitable geographical distribution [2].
Re-eligibility
Rule 144
[reproducing textually a provision of the Charter (Art. 23, para. 2, last sentence)]
A retiring member of the Security Council shall not be eligible for immediate re-election.
Economic and Social Council
Annual elections
Rule 145
[based directly on a provision of the Charter (Art. 61, para. 2, as amended
under General Assembly resolution 2847 (XXVI)). See introduction paras. 23 and 32]
The General Assembly shall each year, in the course of its regular session, elect eighteen members of the Economic and Social Council for a term of three years [3].
Re-eligibility
Rule 146
[reproducing textually a provision of the Charter (Art. 61, para. 2, last sentence)]
A retiring member of the Economic and Social Council shall be eligible for immediate re-election.
Trusteeship Council
Occasions for elections
Rule 147
When a Trusteeship Agreement has been approved and a Member of the United Nations has become an Administering Authority of a Trust Territory in accordance with Article 83 or Article 85 of the Charter, the General Assembly shall hold such election or elections to the Trusteeship Council as may be necessary, in accordance with Article 86. A Member or Members elected at any such election at a regular session shall take office immediately upon their election and shall complete their terms in accordance with the provisions of rule 139 as if they had begun their terms of office on 1 January following their election.
Terms of office and re-eligibility
Rule 148
[based directly on a provision of the Charter (Art. 86, para. l c)]
A non-administering member of the Trusteeship Council shall be elected for a term of three years and shall be eligible for immediate re-election.
Vacancies
Rule 149
At each session the General Assembly shall, in accordance with Article 86 of the Charter, elect members to fill any vacancies.
Explanation: hope this helps you :)