ANS:E - 16 mm

The concrete cover to the main reinforcement bars in a slab is typically specified in construction standards or design codes to ensure adequate protection of the reinforcement from environmental effects such as corrosion and to provide sufficient fire resistance. The concrete cover is measured from the outer surface of the reinforcement to the nearest outer surface of the concrete. For a slab with main reinforcement bars of diameter 16 mm, the concrete cover is typically specified to be at least 15 mm to ensure proper protection and durability of the reinforcement. Therefore, the correct option is: 16 mm