The correct answer is $\boxed{{\text{P}}\sqrt 2 }$.
When two forces, each equal to P, act at 90° to each other, they form a right triangle. The resultant force is the hypotenuse of this triangle, and its magnitude is equal to the square root of the sum of the squares of the two forces. In this case, the two forces are P and P, so the resultant force is:
$${\text{R}} = \sqrt{P^2 + P^2} = \sqrt{2P^2} = {\text{P}}\sqrt 2 $$
Option A is incorrect because it is the magnitude of one of the forces, not the resultant force. Option B is correct because it is the magnitude of the resultant force. Option C is incorrect because it is the magnitude of one of the forces divided by $\sqrt{2}$. Option D is incorrect because it is twice the magnitude of one of the forces.