The sentence option that correctly uses a colon is: B. When Ariana goes shopping, there is only one thing to do: plan to spend the whole day at the mall.
A colon is a punctuation mark that is used to signify the addition of thoughts in a speech. In sentence B, the colon is correctly used because it adds extra thought about what Ariana will be doing.
It can be used to provide further explanation on a matter or include examples or quotations.
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