Final answer:
Steric exclusion in chromatography, referred to in part (d), is the process where large molecules are prevented from entering the pores of the stationary phase, resulting in their earlier elution compared to smaller molecules.
Step-by-step explanation:
The student's question asks to identify the mode of separation in chromatography known as steric exclusion. Steric exclusion, or size exclusion chromatography, is described by option (d), which is the hindrance of large molecules from entering the stationary phase. In chromatographic techniques such as liquid-solid column chromatography, compounds in a sample separate as they pass down the column, based on their interactions with the stationary phase. Some solutes with larger partition coefficients are more likely to interact with the stationary phase, hence taking longer to elute. Components in the mobile phase have different retention times due to their varying interactions with the stationary phase, which can depend on factors like hydrophobicity in reverse phase HPLC or molecular size in steric exclusion.
In steric exclusion chromatography specifically, large molecules are excluded from the pores of the stationary phase due to their size and elute earlier than smaller molecules that can enter the pores and are thus retained longer. This separation technique does not rely on the chemical affinities of the molecules but rather on the physical size exclusion principle, making it particularly useful for the separation of biomolecules like proteins and polymers. The movement of solutes between liquid and solid phases (a), the separation of molecules based on their charge (b), and the attraction of solutes to a solid surface (c), all are different mechanisms of separation not specifically associated with steric exclusion.