Assume that the import of a particular amino acid across the plasma membrane is observed (1) to occur only down its concentration gradient and (2) to slow when a different but similar amino acid is added to the extracellular fluid. The movement of the amino acid through the membrane is most likely by

The correct answer is - facilitated diffusion.

Explanation:

Facilitated diffusion is a form of facilitated transport of molecules such as amino acid, glucose, and others through the plasma membrane involving the passive or indirect movement of molecules along their concentration gradient, which is led by the presence of another molecule normally a pore or channel made up of integral protein.

Facilitated diffusion depends on several factors such as the size of molecules, type of molecules, similarity of molecules to extracellular matrix or fluid that slow or fast the diffusion process.

Thus, the correct answer is - facilitated diffusion.