The sentence that is punctuated correctly is sentence C.
Explanation:
When you use a complete sentence to introduce a quotation, you need a colon after the sentence. If you used a comma, you would create a comma splice. In the sentence above, the sentence introduces the quotation, the words actually said by the speaker's grandmother. That is why the words are inside quotation marks.
The quotation is not a question, so that leaves out option D. In B, the colon is wrongly placed. Finally, the semicolon in A would create a similar error as with the comma. Taking all this into account, it can be said that C is the right choice.
