Final answer:
Blue litmus paper remains blue in the presence of a strong base, as bases cause blue litmus paper to stay blue, indicating the substance is alkaline.
Step-by-step explanation:
Litmus paper that is blue will stay blue in the presence of a strong base. Blue litmus paper is an indicator that is used to test whether a substance is acidic or basic. When blue litmus paper is placed in a strong base, it does not change color. This is because litmus turns blue in the presence of a base, while it turns red in the presence of an acid. Therefore, the correct answer to the question is C. Blue.