If something is described as lucrative, it is primarily:

When something is described as lucrative, it refers specifically to its potential to generate significant financial gain or earnings. This term is often used in contexts where an investment, job, or business opportunity can yield a substantial profit. The focus is on the financial benefits that can be gained, making it a desirable prospect for those seeking economic success.

In contrast, the other options do not capture the essence of "lucrative." While "expensive and rarely bought" refers to high cost rather than profitability, "not valuable" completely contradicts the meaning, as a lucrative opportunity must inherently possess value. Lastly, "favorable and pleasant" suggests positive qualities but does not imply a financial aspect, which is central to understanding the term "lucrative." Thus, the connection to finances or earnings is what makes the choice about financial gains the correct answer.