Which types of molecules make up the majority of a cell's plasma membrane?
a. sugars and starches
b. dna and rna
c. proteins and lipids
d. salts and water

The best answer is C.
Plasma membranes are mainy made up of lipids and proteins. The lipids are glycerophospholipid, molecules composed of glycerol, a phosphate group and two fatty acid chains. Glycerol is a three-carbon molecule that functions as a backbone of these membrane lipids. 
Within each glycerophospholipid, fatty acids are attached to the first and second carbon, while the phophate group is attached to the third carbon of the glycerol backbone.
Proteins such as glycoprotein are attched to the glycerophospholipids in the cell membrane. They are used for cell recognition  and also act as receptors and antigens.