The sentence that displays proper subject-verb agreement is:
"Few people know how to water ski."
The other sentences have subject-verb agreement errors. Here’s a brief explanation of each:
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"The people in line takes life jackets when boarding the boat." - "The people" (plural) should have "take" (not "takes").
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"One of her friends bring gifts for Harper's party." - "One" (singular) should have "brings" (not "bring").
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"All of the musicians is invited to perform for the talent show." - "All" (plural) should have "are" (not "is").
So, the correct answer is: Few people know how to water ski.