The correct answer is: B. 7.0 N/mm2.
A first class brick is a brick that has a crushing strength of not less than 7.0 N/mm2. This means that the brick can withstand a force of 7.0 newtons per square millimeter before it breaks.
The other options are incorrect because they are not the minimum crushing strength required for a first class brick. Option A is 3.5 N/mm2, option C is 10.5 N/mm2, and option D is 14.0 N/mm2.
The crushing strength of a brick is a measure of its ability to withstand compressive forces. It is determined by testing the brick in a compression machine. The brick is placed between two plates and a load is applied until the brick breaks. The crushing strength is then calculated as the force divided by the area of the brick.
The crushing strength of a brick is important because it affects the load-bearing capacity of a structure. A brick with a higher crushing strength can support a greater load than a brick with a lower crushing strength. This is why first class bricks are required for use in load-bearing structures.