For a longitudinal reinforcing bar in a column, the minimum cover shall neither be less than the diameter of bar nor less than A. 15 mm B. 25 mm C. 30 mm D. 40 mm

The correct answer is: C. 30 mm.

The minimum cover for a longitudinal reinforcing bar in a column is 30 mm. This is to ensure that the bar is adequately protected from fire and corrosion. The cover also helps to distribute the load evenly across the concrete.

Option A is incorrect because the minimum cover for a longitudinal reinforcing bar in a column is 30 mm, not 15 mm.

Option B is incorrect because the minimum cover for a longitudinal reinforcing bar in a column is 30 mm, not 25 mm.

Option D is incorrect because the minimum cover for a longitudinal reinforcing bar in a column is 30 mm, not 40 mm.