What does a declarative sentence always end in?
It always states a fact or argument, so it always ends in a period.
From the word declarative, a declarative sentence is a declaration. It is a statement - e.g. "A declarative sentence always ends in a period." It is not a question (?) nor something that is highly expressed (exclamatory, !).
Note: However, this does not mean that all sentences ending with periods are declarative ones. Some may be imperative (commands/requests) like "Please get my books from the locker."