The sentences where the pronoun "who" should be replaced with "whom" are:
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Who do you think should be the next president of the science club? (should be "Whom do you think should be the next president of the science club?")
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The water from the faucet has turned brown, but I'm not sure who to call about it. (should be "The water from the faucet has turned brown, but I'm not sure whom to call about it.")
The other sentences correctly use "who" as a subject pronoun.