Which of the following is a bondline structure of glycerol?

Glycerol is propane-1,2,3-triol, a three-carbon chain with hydroxyl groups at each position: HOCH$_{2}$CH(OH)CH$_{2}$OH.

Option A contains a double bond and three hydroxyl groups—unsaturated, not glycerol.
Option B also contains a double bond and three hydroxyl groups—unsaturated, not glycerol.
Option C shows a four-carbon chain—incorrect carbon count.
Option D correctly depicts a three-carbon chain with hydroxyl groups at positions 1, 2, and 3.

The structure corresponds to HOCH$_{2}$CH(OH)CH$_{2}$OH.