C

To identify the correct quantum numbers for an electron in the 3d subshell, we need to consider the following:

1. **Principal Quantum Number (n)**: For the 3d subshell, n = 3.
2. **Azimuthal Quantum Number (l)**: For d orbitals, l = 2. However, the question mentions the 3d, which indicates that we should be considering a certain sub-level of the d orbitals.
3. **Magnetic Quantum Number (m_l)**: The possible values for m_l in the d sublevel (l = 2) range from -2 to +2. Thus, m_l can be -2, -1, 0, +1, or +2.
4. **Spin Quantum Number (m_s)**: The spin quantum number can either be +1/2 or -1/2.

Given these conditions, let's analyze the options:

- (a) n = 3, l = 1, m_l = +1, m_s = -1/2: Incorrect because l should be 2 for d orbitals.
- (b) n = 3, l = 2, m_l = +2, m_s = -1/2: This one is correct but doesn't match the specifications given by the options expected.
- (c) n = 3, l = 1, m_l = -1, m_s = +1/2: Incorrect for the same reason as (a).
- (d) n = 3, l = 0, m_l = +1, m_s = -1/2: Incorrect since l must be 2 for the d orbital.

Hence the closest match to the quantum numbers for a d electron in the 3d subshell is option **B**, where l = 2.