Which subunit encounters an mRNA to begin the process of translation…

The correct answer is Option (2) → Small subunit of ribosome

The ribosome is the cell's protein factory. In its inactive state it exists as two separate subunits, a large one and a small one. Translation begins when the small subunit meets an mRNA. The large subunit has two sites where the next amino acids bind close enough to form a peptide bond.

Option (1) Large subunit of ribosome — Incorrect. The large subunit holds the two sites for amino acids and helps form the peptide bond, but it is not the subunit that first encounters the mRNA.

Option (2) Small subunit of ribosome — Correct. Translation of the mRNA begins when the small subunit of the ribosome encounters it.

Option (3) Large and small subunits of ribosome — Incorrect. The two subunits are separate in the inactive ribosome. NCERT names only the small subunit as the one that meets the mRNA to begin translation.

NCERT Extract:
Translation – “… The cellular factory responsible for synthesising proteins is the ribosome. The ribosome consists of structural RNAs and about 80 different proteins. In its inactive state, it exists as two subunits; a large subunit and a small subunit. When the small subunit encounters an mRNA, the process of translation of the mRNA to protein begins. There are two sites in the large subunit, for subsequent amino acids to bind to and thus, be close enough to each other for the formation of a peptide bond.”