Answer: Option D. "electrochemical gradient"
Explanation:
Electrical gradient is defined as the electrical potential acts on an ion that drives it in one or another direction.
Concentration gradient is defined as the movement of particles (solutes) from higher concentration area to lower concentration area, separated by a membrane.
The combination of electrical gradient and a concentration gradient is called electrochemical gradient that allows the movement of an ion across a membrane based on solute concentration.
Hence, the correct option is D.
