The maximum thickness of boundary layer in a pipe of radius r is A. 0 B. $$\frac{{\text{r}}}{2}$$ C. r D. 2r

In a pipe of radius $r$, the maximum thickness of the boundary layer occurs at the pipe wall and is given by:

$$\delta = \frac{{\text{r}}}{2}$$

The other options are incorrect because they are either too small or too large. Option A is incorrect because the boundary layer thickness cannot be zero. Option C is incorrect because the boundary layer thickness cannot be equal to the pipe radius. Option D is incorrect because the boundary layer thickness cannot be twice the pipe radius.