Final answer:
Increasing temperature is the least effective way to dampen the lateral diffusion of membrane proteins in lipid bilayers because it increases membrane fluidity rather than reducing it. Option a is the correct answer.
Step-by-step explanation:
When considering the question of which factor is the least effective way to dampen the lateral diffusion of membrane proteins in lipid bilayers, we must look at the impacts of temperature, membrane viscosity, protein size, and cholesterol on the membrane's fluidity.
Increasing temperature generally increases the fluidity and, consequently, the diffusion of proteins within the membrane. Increasing membrane viscosity, which can be achieved by having more saturated fatty acids in the phospholipid tails, would make the membrane more rigid and decrease the rate of protein diffusion. Similarly, increasing the size of the proteins would generally reduce their mobility due to steric hindrance. However, the addition of cholesterol to the membrane serves to restrict movement by fitting in between the fatty acid tails, thereby reducing the fluidity of the membrane and damping the effects of temperature fluctuations on fluidity.
Considering this information, increasing temperature (option a) is the least effective method to dampen the lateral diffusion of membrane proteins because it actually tends to increase the fluidity of membranes rather than decrease it. Therefore, the correct option in this scenario is increasing temperature.