Final answer:
The most acidic substance among the given options is pH 1.
Step-by-step explanation:
In this case, the substance with the lowest pH value, which is pH 1, is the most acidic. The lower the pH value, the more acidic the substance. Therefore, pH 3 is also more acidic than pH 7 and pH 10. In summary, the correct answer is option d. pH 1 is the most acidic.