Final answer:
Unsaturated fatty acids increase the fluidity of the membrane in a phospholipid.
Step-by-step explanation:
The characteristic that increases the fluidity of the membrane in a phospholipid is unsaturated fatty acids. In their unsaturated form, the fatty acids contain double bonds between adjacent carbon atoms, resulting in a bend in the carbon string. This bend creates more space between the fatty acid tails, allowing membrane components to move more freely and increasing the fluidity of the membrane.