Answer:
B molecule.
Explanation:
Receptor may be defined as the molecule to which the ligands or the signalling molecule binds. The binding of the molecules with its receptor helps in initiating the signalling cascade of the body.
The molecule that are marked B can acts as receptor because the receptor may present on the cell membrane or in the cytosol or in the nucleus. The B molecule shows the recess marked with the other molecule.
Thus, the correct answer is B.