The correct answer would be the active transport across the cell membrane.
The main functions of the cell membrane are a barrier, regulates transport (passive, facilitated, and active), the presence of receptors et cetera.
Only active transport (such as sodium pump, calcium pumps et cetera) requires the input of energy (ATP) for proper functioning. It uses energy to pump substances against the concentration gradient.
Thus, only active transport would be affected if less cellular energy is available.
