When cooked sushi rice gets cold in the refrigerator, which change inside the grains most directly makes it feel firmer?

Answer: Starch chains rejoin into ordered networks that make the grain firm.

Starch chains rejoin into ordered networks that make the grain firm. ✓ — Right. Cooling lets gelatinized starch chains reassociate into a more ordered network, a process called retrogradation. Rice texture research connects that process with higher hardness during cooling and storage. Cold sushi rice feels firm because its starch network is moving back toward a more ordered state, not merely because its temperature has dropped.

Surface water leaves the grains and dries them into firmness. — Surface drying can affect the outside of rice, so this answer notices a real change. But the firmness of refrigerator-cold sushi rice comes mainly from starch inside the grain, not from water leaving the surface. Gelatinized starch chains reassociate into ordered structures during cooling, and that internal rearrangement is what rice texture research links to higher hardness.

Grain walls cave in from the cold and squeeze the inside tighter. — The cold can make the grain feel tighter, so this answer catches part of the texture change. But the firming comes mainly from starch inside the cooked grain, not from the outer wall collapsing. As the rice cools, gelatinized starch chains reassociate and form a more ordered network, and rice texture research links that change to higher hardness.