"The fluidity of a biological membrane" is usually higher in plant cell membranes than animal cell membranes. is enhanced by the presence of cholesterol. increases with increasing temperature. is decreased when the proportion of unsaturated fatty acids is increased. describes the movement of water within the membrane double layer.

Of the mentioned statements, 1st and 2nd statements are correct.

Explanation:

With an increase in temperature, the fluidity of a cell membrane increases as the association between the phospholipids gets weakened with the increase in temperature that eventually encourages enhancement in the fluidity of the cell membranes.  

Also, with the increase in the concentration of cholesterol molecules, the fluidity of a biological membrane enhances, that is, at low temperature, cholesterol enhances the fluidity of the membranes by preventing the clustering of the phospholipids present in the membranes, and at greater temperatures, it reduces fluidity as it steadies the membrane and elevates its melting point.