Final answer:
Receptor specificity is influenced by the shape and charge of the receptor, and the hydrophobicity of the ligand, but not directly by the size of the ligand.
Step-by-step explanation:
The question pertains to properties that contribute to receptor specificity in cell signaling. Receptor specificity is determined by various factors, such as the shape of the receptor, the charge of the receptor, and the hydrophobicity of the ligand. An important aspect of receptor-ligand interaction is that hydrophilic ligands generally bind to cell-surface receptors, while hydrophobic ligands are suited for internal receptors. Concerning the choices provided - (a) the shape of the receptor, (b) the charge of the receptor, and (d) hydrophobicity of the ligand - all contribute to receptor specificity. However, (c) the size of the ligand is not mentioned as a factor that affects receptor specificity, but rather it is a characteristic of ligands themselves that can influence whether or not they can bind to a particular receptor.