The correct answer is: B. Smaller than the size of a fully burnt brick.
The internal size of the mould used in brick preparation is smaller than the size of a fully burnt brick. This is because the brick shrinks when it is fired. The amount of shrinkage depends on the type of clay used and the firing temperature.
Option A is incorrect because the internal size of the mould is smaller than the size of a fully burnt brick.
Option C is incorrect because the internal size of the mould is smaller than the size of a fully burnt brick.
Option D is incorrect because the internal size of the mould is smaller than the size of a fully burnt brick.