Final answer:
Martha's plan to invest her bonus in the stock market falls under the subject of Business, specifically asset allocation and diversification.
Step-by-step explanation:
The subject of this question is Business and the grade level is High School.
Martha is considering investing a portion of her recent bonus in the stock market. This falls under the realm of asset allocation, which involves dividing investments among different asset classes such as stocks, bonds, and cash. By investing in the stock market, Martha is diversifying her investment portfolio, which helps reduce the level of risk. It is important to note that the stock market's returns can be variable, and investing in a wide range of companies can help mitigate this risk.