B

Step 1: The maximum speed of a car in a circular path is determined by the formula: \( v_{max} = \sqrt{g \cdot r \cdot \mu} \) where:
- \( v_{max} \) is the maximum speed
- \( g \) is the acceleration due to gravity (approximately 9.8 m/s²)
- \( r \) is the radius of the circular path
- \( \mu \) is the coefficient of friction.

Step 2: When the road is dry, the maximum speed is given as 10 m/s:
\( 10 = \sqrt{g \cdot r \cdot \mu} \)

Squaring both sides, we get:
\( 100 = g \cdot r \cdot \mu \)

Step 3: When the road is wet, the new coefficient of friction is \( \mu' = 0.4 \cdot \mu \).
Thus, the new maximum speed is:
\( v' = \sqrt{g \cdot r \cdot \mu'} = \sqrt{g \cdot r \cdot 0.4 \cdot \mu} \)

Step 4: Since we also know that \( g \cdot r \cdot \mu = 100 \):
\( v' = \sqrt{0.4 \cdot 100} = \sqrt{40} = 6.32 \text{ m/s} \)

Therefore, the maximum speed permitted on a wet road is approximately 6.32 m/s.