Shear stress on the wall of the boundary layer can be determined using the velocity gradient and dynamic viscosity of the fluid. Let's break down the given options and determine the correct answer.

a) μθ
b) μ θ
c) μ/θ
d) θ/μ

To find the correct answer, we need to understand the relationship between shear stress, velocity gradient, and dynamic viscosity.

1. Shear stress (τ): Shear stress is the force per unit area acting parallel to the surface. In fluid dynamics, it is caused by the velocity gradient.

2. Velocity gradient (θ): The velocity gradient measures the rate of change of velocity with respect to distance in the fluid. It gives an indication of how the fluid velocity changes near the wall of the boundary layer.

3. Dynamic viscosity (μ): Dynamic viscosity is a measure of a fluid's resistance to flow. It quantifies the internal friction within the fluid.

Now let's analyze the options:

a) μθ: This option correctly represents the product of dynamic viscosity (μ) and velocity gradient (θ). Since shear stress is directly proportional to both factors, the answer is a) μθ.

b) μ θ: This option incorrectly represents the product of dynamic viscosity (μ) multiplied by the velocity gradient (θ). The correct representation is μθ, not μ θ.

c) μ/θ: This option represents the division of dynamic viscosity (μ) by the velocity gradient (θ). However, the shear stress is given by the product of μ and θ, not their division. Therefore, this option is incorrect.

d) θ/μ: This option represents the division of the velocity gradient (θ) by the dynamic viscosity (μ). The shear stress is given by the product of μ and θ, not their division. Thus, this option is also incorrect.

Therefore, the correct answer is a) μθ, as it accurately represents the shear stress on the wall of the boundary layer in the direction of motion.