The answer, unfortunately, is d), diseases can be spread by all of the above!
However, that doesn't mean that all diseases can be spread by all of these methods. Some diseases, such as HIV, can only be spread by the direct exchange of bodily fluids- usually through unprotected sexual contact, sharing intravenous needles, or through pregnancy. You cannot "catch" HIV simply by coming into contact with someone who has the virus, or because they cough or sneeze near you.
In contrast, Influenza (the "flu") can be spread in exactly this way, through "respiratory" droplets". It can also be spread by indirect contact, by touching an object that has been touched by someone with the flu, and then touching your own mouth, eyes or nose. It is thought that influenza germs can live on surfaces for up to 8 hours!
Cholera is an example of a disease which is spread most commonly through infected food and water, and has thrived in hot, unsanitary conditions in places such as Africa and India.