In this sentence, WHO = World Health Organization * World Health Organization's objective is the attainment by all people of the highest possible level of health. Attainment - the action or fact of achieving a goal toward which one has worked. In this sentence, attainment refer's to the World Health Organization's goal of achieving the highest possible level of health for all people. * World Health Organization's objective is the approach by all people of the highest possible level of health. This sentence is not the answer because it would be better written if "of" was changed to "to": World Health Organization's objective is the approach by all people to the highest possible level of health. Even with the change, attainment is better. * World Health Organization's objective is the advance by all people of the highest possible level of health. Again, "of" changes to "to" would be better, but still, attainment fits best. * World Health Organization's objective is the acquisition by all people of the highest possible level of health. This is the second best sentence in my opinion, but attainment is better for two reasons: 1) attainment is often used when talking about a goal, or objective 2) the sentence structure would be better like the below: World Health Organization's objective is the acquisition of the highest possible level of health by all people.

Also, important to remember: This is a typical exam question testing your knowldge of COLLOCATION. You can read about it here:http://www.proprofs.com/quiz-school/story.php?title=test-your-english-14-collocations The only word from the list which goes well with "level" is "attainment", since in English you ATTAIN a given LEVEL.