Select the incorrect form of the verb.

The correct answer is strucken.

Let's analyze the verb strike and its various forms:
1. Base Form (Infinitive): strike (e.g., "They strike a deal.")
2. Past Simple: struck (e.g., "The clock struck midnight.")
3. Past Participle: The standard past participle is struck (e.g., "She was struck by lightning"). An older or specialized past participle form is stricken, which is still commonly used as an adjective (e.g., "a poverty-stricken area" or "grief-stricken").
4. Incorrect Form: strucken is grammatically incorrect and does not exist as a standard form of the verb strike in modern English.

Therefore, strucken is the incorrect form of the verb.