The gist of George Berkley's philosophy is mind-dependence. Objects and concepts are do not exist, nor are put to use, unless they have been given a specific purpose and use. The purpose and use of objects depends on the human mind because these objects, or concepts, came to exist through human agency.
In Berkley's own words in Principles:
It is evident to any one who takes a survey of the objects of human knowledge, that they are either ideas actually imprinted on the senses, or else such as are perceived by attending to the passions and operations of the mind...
Therefore, things do not merely "come to be", but they actually go through a process of conceptualization that can only occur through the application of all of our senses. Moreover, Berkley explains,
for example, a certain colour, taste, smell, figure and consistence having been observed to go together, are accounted one distinct thing, signified by the name apple.  sensible things; which, as they are pleasing or disagreeable, excite the passions of love, hatred, joy, grief, and so forth.
Therefore, the answer to your question, an operation of the mind is the process of discerning, inductively and deductively, the usage of something. The final product will instill emotion. The emotion in itself is not a part of the creation, hence, grief is not an operation, but a reaction. A loud bang is a consequence of an action and we have no control over its creation, nor is it an operation of the mind. That is also to be ruled out.
We are left with the brick wall, and imagining. While the brick wall is a concept that someone definitely had to had brought from the abstract to the concrete for a specific use, it may or may not instill an idea, or inspire emotion. The object is, according to Berkley, himself, entirely dependent of the meaning and importance that we give it.
Therefore, following Berkley's words, the operation of the mind is definitely "imagining". When we image we breakthrough the first steps of conceptualization and creation. Imagining things allows us to give sensory relevance to things that we may need to create, such as the brick wall, for example. When we imagine, we take the steps to bring the abstract to the concrete.
“we have some notion of soul, spirit, and the operations of the mind, such as willing, loving, hating, in as much as we know or understand the meaning of those words”