Every plain statement or direct instruction in English needs a punctuation mark to signal that the sentence is finished. That mark is the period. The train left on time is a statement, and Take your seat is an instruction, and both are properly closed with a period rather than any other mark.
The alternatives all do different jobs. A semicolon sits inside a sentence to connect two related clauses. A comma is used for pauses and lists within a sentence, not to end it. A dash creates emphasis or inserts a side comment but is not a closing mark for ordinary statements or commands.
Because ending declarative and imperative sentences is specifically the period's job, the answer is option 4.