Fluidity of phospholipid bilayer is increased by

(a) Cholesterol at certain temperatures

(b) Double bonds between carbon atoms in the fatty acid tails

(c) Shorter hydrophobic tails

(d) All the above

All the above.

Explanation:

The cell membrane or plasma membrane is made up of lipid bilayer and proteins are inserted in the bilayer. The cell membrane allows the passage of restricted molecule inside and outside of the cell.

The mobility of the membrane can be increased by the level of cholesterol. The high temperature reduces the mobility of layer and at low temperature the cholesterol level increases the membrane mobility. The double bonds and shorter hydrophobic tails also increases the fluidity of the membrane.

Thus, the correct answer is option (d).