Complete the sentence.
_______ register with a valid email address.

"First, please" is correct. The comma after our introductory word is grammatically correct as it sets off our introductory adverb from the rest of the sentence.
"First please" is incorrect. This answer choice does not have any punctuation and, thus, does not differentiate the introductory adverb first from the rest of the sentence.
"First: please" is incorrect. This answer choice has a colon after our introductory adverb. A colon is meant to set apart two independent clauses, usually in a definition or informative format.
"First; please" is incorrect. This answer choice features a semicolon after our introductory adverb. Semicolons are used to separate independent clauses in a sentence which is not the case here.