The correct option is
C Galileo said, "The earth moves around the sun."
Capitals are used to begin a sentence. All the given sentences begin with a capital letter. Proper names begin with a capital letter. 'Galileo' is a proper name. The letter 'g' of 'Galileo' is capitalised in all the options.
Inverted commas ("...") are used to enclose the exact words of a speaker or a quotation. The words, "The Earth moves around the sun" are the exact words of Galileo. Hence, these needs to be enclosed in inverted commas. In options A, B and D, this is correctly followed. However, in option C, the entire sentence (including the name of the speaker 'Galileo') is enclosed in inverted commas. Hence option C is incorrect.
The given sentence is an assertive sentence, which needs to end with a full stop. Option A ends with a question mark. Option B ends with an exclamation mark.Hence, options A and B are incorrect.
The only options which is correctly punctuated as explained above is option D, which is the correct answer.