Introduction:
In the story "The Ironmaster's Mistress" by Elizabeth Gaskell, the ironmaster is a wealthy man who falls in love with a woman of lower social standing, the peddler's daughter. However, he soon realizes his mistake when the peddler changes clothes and cleans his face, revealing his true identity.

Explanation:
1. When the peddler speaks:
The ironmaster does not immediately realize his mistake when the peddler speaks. In fact, he is initially captivated by the peddler's daughter who is mute and communicates through sign language. The ironmaster is drawn to her beauty and innocence, but he does not realize the deception until later.

2. When he sees an old photograph:
The ironmaster does not see an old photograph in the story, so this option is incorrect. It is important to carefully read and analyze the given options to determine the correct answer.

3. When the peddler changes clothes and cleans his face:
The correct answer is option 'C' - when the peddler changes clothes and cleans his face. This is the moment when the ironmaster realizes his mistake. The peddler, who has been disguised as a woman, reveals his true identity by changing his appearance. The ironmaster is shocked and betrayed to discover that the woman he thought he loved is actually a man.

4. All of these:
The ironmaster does not realize his mistake when the peddler speaks or when he sees an old photograph, so this option is incorrect. The correct answer is option 'C' - when the peddler changes clothes and cleans his face.

Conclusion:
In Elizabeth Gaskell's story "The Ironmaster's Mistress," the ironmaster realizes his mistake when the peddler changes clothes and cleans his face, revealing his true identity. This moment is a turning point in the story, as the ironmaster is faced with the truth and must confront the consequences of his actions. The story serves as a cautionary tale about the dangers of deception and the importance of looking beyond appearances.