**Explanation:**

In a step-up transformer, the primary winding has fewer turns than the secondary winding. As a result, the output voltage is higher than the input voltage. The purpose of providing an iron core in a step-up transformer is to increase the magnitude of mutual flux and thereby increase the voltage transformation ratio.

**Magnitude of Mutual Flux:**

The iron core provides a path for the magnetic flux to flow through. The magnetic flux is generated by the alternating current flowing through the primary winding. The iron core enhances the magnetic field by concentrating and directing the flux lines. This increased magnetic flux links both the primary and secondary windings, resulting in efficient energy transfer from the primary to the secondary.

**Increase in Voltage Transformation Ratio:**

The iron core increases the magnitude of the mutual flux, which in turn increases the voltage transformation ratio. The voltage transformation ratio is the ratio of the secondary voltage to the primary voltage. With a larger mutual flux, the voltage transformation ratio can be increased, allowing for a higher output voltage.

**Decrease in Magnetizing Current:**

The iron core also helps to decrease the magnitude of the magnetizing current. The magnetizing current is the current required to establish the magnetic field in the transformer. By providing an iron core, the magnetic flux is concentrated and the reluctance of the magnetic path is reduced. This leads to a decrease in the magnetizing current, resulting in improved efficiency and reduced power losses.

**Conclusion:**

In conclusion, the purpose of providing an iron core in a step-up transformer is to increase the magnitude of the mutual flux, which in turn increases the voltage transformation ratio. Additionally, the iron core helps to decrease the magnetizing current, resulting in improved efficiency. Therefore, the correct answer is option 'C' - to decrease the magnitude of the magnetizing current.