Solution

Low temperatures can cause a membrane to enter a gel-like phase with very high membrane-lipid viscosity, whereas high temperatures can cause a membrane to become "hyper fluid" with very little viscosity. The temperature dependence of biological membrane. The greater the proportion of unsaturated carbon-carbon bonds (double-bonds) in a lipid's fatty acid molecules, the easier solidification of it occurs, in cold temperature and thus the lesser the changes in plants membrane at lower temperature.
So, the correct answer is 'Increasing the frequency of unsaturated fatty acyl chains'.