Coagulating power of an iron for a colloidal solution depends on:

Its size and magnitude of charge both

The correct answer is option 4. Its size and magnitude of charge both.

The coagulating power of a substance for a colloidal solution refers to its ability to cause the colloidal particles to come together and form larger aggregates, ultimately leading to precipitation or settling. This process is known as coagulation or flocculation.

The coagulating power depends on various factors, but in the context of iron, it primarily depends on its ability to interact with the colloidal particles and promote aggregation. Let's analyze the options:

1. Its magnitude of charge only: The magnitude of charge on iron particles may influence their interaction with charged colloidal particles, but it's not the sole determinant of coagulating power. Other factors, such as size and hydration tendency, also play significant roles.

2. Its size only: While the size of iron particles may affect their interaction with colloidal particles, it's not the only factor determining coagulating power. Other factors, such as charge and hydration tendency, are also crucial.

3. Its tendency to hydrate: The tendency of iron to hydrate can indeed contribute to its coagulating power. Hydration of iron ions can lead to the formation of hydroxide complexes, which may adsorb onto colloidal particles and promote coagulation.

4. Its size and magnitude of charge both: This option captures the comprehensive nature of factors influencing coagulating power. Both the size and magnitude of charge of iron particles can affect their ability to interact with colloidal particles and promote coagulation.

Considering the above analysis, option 4, "Its size and magnitude of charge both," appears to be the most appropriate. The coagulating power of iron for a colloidal solution is influenced by both its size and magnitude of charge.