ANS:B - increase gradually.

In a boundary layer developed along the flow, the pressure decreases in the downstream direction due to the effects of friction and viscous forces. According to Bernoulli's principle, where the pressure in a fluid decreases as the velocity increases, this decrease in pressure in the boundary layer causes the boundary layer thickness to increase gradually in the downstream direction. As the pressure decreases along the flow direction, the velocity gradient within the boundary layer changes, resulting in a thicker boundary layer. This gradual increase in boundary layer thickness is characteristic of boundary layers developing along the flow direction under typical conditions. Therefore, the correct answer is: increase gradually.