Ekistics is a specific academic term, so let's look at what each option claims it means.
The water sports related study: There is no branch of study by this name linked to water sports, so this option does not match the term.
The science of Body Mass: The study of body measurements and mass is closer to anthropometry, not Ekistics, so this is wrong.
The art of card tricks: This has nothing to do with settlements or planning, so it is clearly wrong.
The science of human settlements: Ekistics is the term coined by the Greek architect Constantinos Doxiadis for the science of human settlements, covering how people design and organize towns and cities.
Since Ekistics was created specifically to describe the study of human settlements, option D is the correct choice.
Let's summarize:
Ekistics means the science of human settlements, a term coined by Constantinos Doxiadis.