Which structures add to the fluidity of the plasma membrane:
(a) carbohydrates
(b) atp
(c) glycocalyx
(d) cell cortex

(a) Carbohydrates are the structures  which add to the fluidity of the plasma membrane.

Explanation:

The principle components of plasma membrane are lipids, proteins and carbohydrates which are attached to some proteins and lipids. Membrane fluidity means how much viscous is the lipid bilayer of cell membrane? The membrane phospholipids absorbs fatty acids of different lengths and saturation. When there is amore distance between phospholipids fluidity increases.Another one is the  temperature and cholesterol. So these are the three factors which affects the fluidity of the membrane. The rate of saturated and unsaturated fatty acids decides the fluidity of the membrane at cold temperatures. Cholesterol take up the tasks as buffer blocking the lower temperatures from fluidity inhibition and blocking higher temperatures from increasing of fluidity.