The correct option is
A World Population Day
World population day is observed on 11th July. It creates awareness about the growing population amongst the people. Population issues like family planning, gender equality, poverty, maternal health and human rights are the main target.
No tobacco day is observed on 31st may.
World environment day is observed on 5th June.
World health day is observed on 7th April.
So, the correct answer is option A.