Step 1: Formula for the Radius of the Circular Path.
The radius \( r \) of the circular path of a charged particle in a magnetic field is given by:
\[
r = \frac{mv}{qB}
\]
where \( m \) is the mass of the particle, \( v \) is the velocity, \( q \) is the charge, and \( B \) is the magnetic field strength.
Step 2: Analyzing the Proportionality.
We can express momentum \( p \) as \( p = mv \), so the radius becomes:
\[
r = \frac{p}{qB}
\]
Therefore, the radius is directly proportional to the momentum \( p \) of the particle.
Step 3: Conclusion.
Hence, the correct answer is (B) momentum of the particle.