Solution:

Definition of 'coalesce':
Coalesce means to come together and form one mass or whole. It refers to the process of merging or uniting different elements or parts into a single entity.

Antonym of 'coalesce':
The antonym of 'coalesce' is a word that represents the opposite meaning or action. In this case, the correct antonym is 'separate', which means to divide or break apart.

Explanation:
To understand why 'separate' is the correct antonym of 'coalesce', let's look at the definitions of both words and their opposite meanings:

- Coalesce: To come together and form one whole.
- Separate: To divide or break apart.

When something coalesces, it means that different elements or parts are merging or uniting to form a single entity. On the other hand, when something separates, it means that the previously merged or united parts are being divided or broken apart.

Therefore, 'separate' is the opposite action of 'coalesce', as it involves the act of dividing or breaking apart rather than merging or uniting.

Options:
Let's examine the other options given to see if they can be considered as antonyms of 'coalesce':

- Adhere: Adhere means to stick or cling to something. While it may represent a different action from 'coalesce', it does not directly imply the opposite meaning. Therefore, option 'B' is not the correct antonym.
- Cleave: Cleave means to split or divide something, which might seem like an antonym at first. However, 'cleave' can also mean to adhere strongly to something, which is not the opposite meaning of 'coalesce'. Therefore, option 'C' is not the correct antonym.
- Amalgamate: Amalgamate means to combine or merge different elements or parts into a single entity, which is similar to 'coalesce'. Therefore, option 'D' is not the correct antonym.

Conclusion:
Based on the definitions and opposite meanings of the given options, the correct antonym of 'coalesce' is 'separate' (option 'A'). It represents the opposite action of merging or uniting, as it involves dividing or breaking apart.