Final answer:
The most speculative Moody's rating in the investment grade category is BA (Baa in Moody's terms), which is the lowest investment grade rating, indicating a moderate credit risk. Option D is correct.
Step-by-step explanation:
The most speculative Moody's rating in the investment grade category is BA (also known as Baa). Moody's rates bonds based on their credit risk, with AAA being the highest quality and C being the lowest. Investment grade bonds are considered low to moderate credit risk and include ratings from Baa/BA (Moody's/S&P) up to Aaa/AAA. On the Moody's scale, ratings below Baa are considered non-investment grade or high-yield/junk. Therefore, a Baa/BA rating is the lowest investment grade rating, indicating it is the most speculative within the investment grade category.