Final answer:
The saturation of fatty acids in the phospholipid tails contributes to the fluidity of the membrane.
Step-by-step explanation:
The characteristic of a phospholipid that contributes to the fluidity of the membrane is the saturation of fatty acids in the phospholipid tails. In their saturated form, the fatty acids are straight, whereas unsaturated fatty acids, which contain double bonds, result in a bend in the carbon string. This creates more space between the fatty acid tails, allowing membrane components to move more freely and making the membrane more fluid.