Which of the following is a symptom of colic?

Rolling is a classic symptom of colic in horses. Colic refers to abdominal pain, which can arise from various causes like gas buildup, blockage, or changes in gut motility. When a horse is in pain from colic, it may attempt to alleviate the discomfort through behaviors such as rolling. This movement can be a natural response as the horse tries to find a more comfortable position.

Leaning against objects can occur, but it is often associated with other conditions, and while it may indicate discomfort, it is not as specific to colic as rolling is. Frequent urination does not typically relate to colic and might suggest other health issues. Sudden weight gain is generally not a symptom of colic; instead, it could indicate overeating or other metabolic concerns. Therefore, rolling stands out as a critical behavior indicating that a horse may be suffering from colic.