Final answer:
The letter found under the old painter's door mat is from a secret admirer.
Thus, the correct option is C.
Step-by-step explanation:
The answer to the question is c. A letter from a secret admirer.
This information is not stated directly in the provided texts, but it can be inferred from the description of the letter under the old painter's door mat. In the first passage, the narrator describes finding a solitary letter with a large black seal addressed to the minister in a diminutive female hand. It is also mentioned that the letter was thrust carelessly into one of the compartments of a card-rack hanging from the mantel-piece.
Based on these details, it can be concluded that the letter found under the old painter's door mat is from a secret admirer.
Therefore, the correct option is c. A letter from a secret admirer.