Who said "Inclusion of any matter in the agenda of General Assembly, discussion on it, its study or the…

The words are those of Professor Quincy Wright, who argued in International Law and the United Nations (1960) that placing a matter on the General Assembly agenda, discussing or studying it, or passing a general resolution on it does not amount to "intervention" barred by Article 2(7) of the UN Charter. So the answer is "Prof. Quincy Wright". Hans Kelsen and Oppenheim wrote on the Charter and on intervention, but this formulation is not theirs.