Correct Answer: briefly

Solution : The correct option is the first option.

Explanation:
Briefly (adverb) means doing something in a short or concise manner, summarising without going into great detail. "We considered (in brief) the long-term solution to the problem." Here, in brief refers to doing something quickly or concisely, which aligns with the context of considering the solution briefly or in a concise manner.

The meanings of the other options are as follows:

• Briefless (adjective) refers to being without briefs or lacking in documentation.
• Brief (adjective) means short in duration, concise, or not lasting for a long time.
• Briefness (noun) refers to the quality of being brief or short in duration.

Therefore, the correct sentence would be: "We considered briefly the long-term solution to the problem."