Answer:
The sentence, "The contract is legal," is a complete sentence. It has a noun and a verb.
"Since we entered ..." is not a complete sentence because the "Since" is a conjunction and means that the phrase is not a complete sentence.
"Only if the terms are reasonable," is not a complete sentence, but it would be if you cut out "Only if."
"At any time of the year except spring," has no verb, so it isn't a complete sentence.