B

Step 1: Consider the nature of cathode rays, which are streams of electrons.
Step 2: When these electrons enter a magnetic field at right angles, they experience a magnetic force perpendicular to their velocity (due to the Lorentz force, which is given by the equation \( F = q(v \times B) \)).
Step 3: This force causes the electrons to undergo circular motion because the force acting on them is always directed towards the center of the circular path.
Step 4: If the magnetic field is uniform and the velocity of the electrons is also constant, the radius of the circle can be determined using the formula for centripetal force and magnetic force balance.
Step 5: Therefore, the trajectory of cathode rays in a magnetic field when projected at right angles is a circle.
Hence, the correct answer is B.