Which of the following agents can be used as a laxative?

Docusate sodium is classified as a stool softener and is used as a laxative to help relieve constipation. It works by increasing the amount of water and fats in the stool, making it easier to pass. This mechanism helps to prevent straining during bowel movements, especially in individuals who may have difficulty with regular elimination due to pain or other conditions.

In contrast, aluminum hydroxide is primarily an antacid used to neutralize stomach acid and is not designed for use as a laxative. Simethicone is an anti-foaming agent used to alleviate gas and discomfort caused by excess gas in the gastrointestinal tract, rather than addressing constipation. Diphenoxylate is an opioid antidiarrheal agent, used to treat diarrhea, and it works by slowing down gut motility, which would not relieve constipation.

Thus, docusate sodium stands out as the agent specifically intended for easing constipation, making it the appropriate choice among the options provided.