Here are the answers to your questions:

1. **C. headache** - The underlined part is pronounced differently compared to the others. In "ensure," the "s" is pronounced as /ʃ/, in "shouting," the "sh" is /ʃ/, and "machine" has the "ch" pronounced as /ʃ/, while in "headache," the "ch" is pronounced as /tʃ/.

2. **A. enter** - The "en" has a different vowel sound compared to the long vowels in "many," "measure," and "sacrifice."

3. **C. breakup** - The stress pattern is different. "Interested," "ridicule," and "conserve" have stress on the second syllable, while "breakup" has stress on the first syllable.

4. **C. policeman** - "Marmalade," "marathon," and "fasten" have stress on the first syllable, whereas "policeman" has stress on the second syllable.

5. **A. I can’t remember whether or not I locked the door.** - This response logically follows the question about going back, providing a reason for returning.

6. **B. Would you like some more turkey?** - This is a natural follow-up to hosting a party, asking if the guest would like more food.