C

Step 1: Consider the properties of a charged particle moving in a magnetic field. When a charged particle with charge q moves in a magnetic field $ extbf{B}$, it experiences a magnetic force given by the Lorentz force equation:

$$\textbf{F} = q(\textbf{v} \times \textbf{B})$$
where $\textbf{v}$ is the velocity vector of the particle.

Step 2: Analyze the motion based on the angle θ. The angle θ is defined between the velocity vector of the charged particle and the direction of the magnetic field.
- If θ = 0 or θ = 180° (particle moves parallel or anti-parallel to the field), the magnetic force is zero, and the particle moves in a straight line.
- If 0 < θ < 90° or 90° < θ < 180°, the velocity can be resolved into two components: one parallel to the magnetic field and one perpendicular to it. The perpendicular component will experience a force that causes circular motion, while the parallel component ensures that the particle continues moving along the field direction.
Step 3: The combination of circular motion due to the perpendicular component and linear motion due to the parallel component results in a helical path.
Therefore, the correct answer is: C - helical.