The shape of guard cells changes due to a change in the amount of water in cells.
Explanation: When guard cells take in water, they swell and become turgid, causing the stomatal pore to open. This change in water content is what drives the shape change in the guard cells.
Why other options are incorrect:
A – protein composition of cells:
While protein composition can play a role in cell function, it’s not the primary driver of shape change in guard cells. The shape change is primarily due to water uptake.
B – temperature of cells:
Temperature can influence some cellular processes, but its direct effect on guard cell shape is negligible compared to water content.
D – position of nucleus in the cells:
The position of the nucleus doesn’t directly impact the shape of guard cells. Their shape is determined by turgor pressure related to water content.