Step 1: Sort the four bodies by mandate.
NASA: space. UNESCO: education/culture. UNO: the overall parent organisation. WHO: health.
Step 2: Match "mental health" to a mandate.
Mental health is a health subject, so it falls under the one agency whose entire purpose is public health.
Step 3: Confirm with a known example.
WHO's mental health programs, including its work on depression and suicide prevention, are well documented as WHO-led initiatives.
Final Answer:
The health-focused agency, WHO, is the correct answer.