S.I. unit for dipole moment is

coulomb-metre

Explanation:

The dipole moment is a measure of the separation of positive and negative charges within a system, indicating the polarity of a molecule. It is defined as the product of the magnitude of the charge (Q) and the distance (d) between the centers of positive and negative charge:

Dipole Moment (μ) = Q × d

In the International System of Units (SI), the unit of charge (Q) is the coulomb (C), and the unit of distance (d) is the meter (m). Therefore, the SI unit for dipole moment is the coulomb-meter (C·m). This corresponds to option C.

In practice, especially in atomic and molecular physics, the dipole moment is often expressed in Debye units (D) for convenience. The Debye is a non-SI unit where:

1 D ≈ 3.336 × 10⁻³⁰ C·m

It's important to note that while Debye is commonly used, the standard SI unit remains the coulomb-meter.

Regarding the other options:

• A. e.s.u-cm: This represents the electrostatic unit of charge (esu) multiplied by centimeters, a unit used in the centimeter-gram-second (CGS) system, not in SI.
• B. Coulomb-cm: While coulomb is an SI unit of charge, centimeter is not the standard SI unit of length; the correct SI unit is meter.
• D. e.s.u-meter: This combines a CGS unit of charge (esu) with an SI unit of length (meter), which is inconsistent.

Therefore, the correct SI unit for dipole moment is the coulomb-meter (C·m), making option C the correct choice.