ANS:A - outside

Boiler tube size is specified by its thickness and outside diameter. The outside diameter of a boiler tube is an important parameter for specifying its size, as it determines the dimensions of the tube and how it fits within the boiler system. The outside diameter is measured from the outer surface of the tube across its widest point. Thickness, on the other hand, refers to the wall thickness of the tube, which is also a critical factor in determining the strength and durability of the tube. Therefore, boiler tube size is typically specified by its thickness and outside diameter.