The stock market has risen ____________ in the last six months and now is a good time to invest.

The choice "considerably" is the most appropriate word to fill in the blank because it conveys a sense of significant increase in the stock market over the last six months. Using "considerably" indicates that the rise is substantial enough to warrant attention and suggests that this positive trend makes it a good opportunity for investment.

In contrast, the other options imply different levels of change that would not align with the context provided. "Consistently" suggests regularity without indicating magnitude, while "insignificantly" and "marginally" imply small or trivial increases, which would not support the statement that it is a good time to invest. These options do not convey the necessary strength of the market's rise needed to encourage investment, making "considerably" the best choice.