The losses in a transformer can be categorized into three types: copper loss, hysteresis loss, and eddy current loss. The constant power loss in a transformer is given by the combination of hysteresis loss and eddy current loss.

Copper Loss:
- Copper loss occurs due to the resistance of the copper windings in the transformer.
- When current flows through the windings, it encounters resistance, resulting in the conversion of electrical energy into heat.
- Copper loss is directly proportional to the square of the current flowing through the windings and can be minimized by using thicker wires with low resistance.

Hysteresis Loss:
- Hysteresis loss occurs due to the magnetic properties of the transformer core material.
- When alternating current flows through the windings, the magnetic field in the core constantly changes direction, causing the core material to magnetize and demagnetize.
- This process results in energy loss due to the internal friction within the core material, which is converted into heat.
- Hysteresis loss is directly proportional to the frequency and volume of the magnetic flux and can be reduced by using materials with low hysteresis coefficients.

Eddy Current Loss:
- Eddy current loss occurs due to the circulating currents induced in the conducting materials, such as the transformer core, by the varying magnetic field.
- These circulating currents create a magnetic field that opposes the original field, resulting in energy loss in the form of heat.
- Eddy current loss is directly proportional to the square of the thickness of the conducting material, the frequency of the magnetic field, and the resistivity of the material.
- It can be minimized by using laminated cores made of materials with high resistivity.

Constant Power Loss:
- The constant power loss in a transformer is given by the sum of hysteresis loss and eddy current loss.
- This power loss is independent of the load on the transformer and represents the energy dissipated as heat in the core.
- It is usually expressed as a percentage of the transformer's rated power and is an important factor in determining the overall efficiency of the transformer.

Based on the above explanations, it can be concluded that the constant power loss in a transformer is determined by the hysteresis loss and eddy current loss, which makes option B the correct answer.