What Does the Model Termed "Lock-And-Key" Stand For

What does the model termed "lock-and-key" stand for?

A) The hydrogen bonds in the α-helix fit each other like a key fits a lock.
B) The amino acids in a peptide chain fit each other like a key fits a lock.
C) The substrate fits the active site of the enzyme like a key fits a lock.
D) The nonpolar sites of proteins fit each other like a key fits a lock.
E) The N-terminal and the C-terminal ends of a protein fit each other like a key fits a lock.