What type of transport uses energy from one molecule's movement to transport another against its gradient?

The correct answer is rooted in the concept of secondary active transport, which is an essential mechanism in cellular physiology. Secondary active transport does not use ATP directly, but instead utilizes the energy created from the movement of one molecule down its electrochemical gradient to drive the movement of another molecule against its gradient.

In this process, a primary active transport system first establishes a gradient, usually through the use of ATP to pump ions like sodium (Na+) or hydrogen (H+) out of the cell. Then, as these ions move back into the cell, often through specific transport proteins, they harness their kinetic energy to facilitate the co-transport of another substance, such as glucose or amino acids, against its concentration gradient. This method is crucial for nutrient uptake in many cells, particularly in the intestines and kidneys.

Other options like passive transport and facilitated diffusion do not use energy to move substances against their gradients but rather allow movement down a concentration gradient without any added energy expenditure. Synthetic diffusion is not a recognized biological transport process, which eliminates it as a valid choice. Thus, the concept of harnessing energy from one molecule's movement to assist another is what defines secondary active transport accurately.