Introduction:
Receptors are proteins present on the surface of cells or within cells that bind specifically to particular signaling molecules called messengers. The selectivity of receptors refers to their ability to recognize and bind only to specific messengers while ignoring others. This selectivity is crucial for proper cell signaling and communication. There are several factors that contribute to the selectivity of receptors towards messengers.

Explanation:
The correct answer is option 'D' - Location in the membrane. The location of receptors in the membrane does not play a significant role in their selectivity towards messengers. Let's discuss the other factors in detail:

a) Shape of binding site:
The shape of the binding site on the receptor is a crucial factor in determining selectivity. The binding site is complementary in shape to the messenger molecule, much like a lock and key. Only messengers with a specific shape can fit into the binding site and form stable interactions with the receptor. This ensures that only the appropriate messengers can activate the receptor.

b) Structure:
The overall structure of the receptor determines its selectivity towards messengers. Receptors possess specific amino acid residues that form the binding site. The arrangement and composition of these residues contribute to the receptor's ability to recognize and bind to specific messengers. Changes in the receptor structure can alter the selectivity and affinity for different messengers.

c) Amino acid composition:
The amino acid composition of the receptor's binding site is critical for selectivity. Different amino acids have different chemical properties and side chains, which can form specific interactions with the messenger molecule. The presence of specific amino acids in the binding site allows the receptor to selectively recognize and bind to certain messengers while excluding others.

d) Location in the membrane:
The location of the receptor in the membrane does not directly influence its selectivity towards messengers. Receptors can be found on the cell surface or within the cell, and their selectivity is primarily determined by factors such as the shape of the binding site, receptor structure, and amino acid composition.

Conclusion:
The selectivity of receptors towards messengers is determined by various factors, including the shape of the binding site, receptor structure, and amino acid composition. The location of the receptor in the membrane does not play a significant role in selectivity. Understanding the factors that contribute to receptor selectivity is essential for comprehending cell signaling and developing targeted therapies for various diseases.