Hi Sara, it is "devient" because in a sentence like "la situation devient..." the subject is in third person singular (he/she/it) and the third person singular of the verb "devenir" is "devient".
Incidentally, to check any verb conjugation, you might find this site helpful: http://conjugator.reverso.net/conjugation-french-verb-devenir.html
For "devenir" in present tense indicative it goes like this:
je deviens,
tu deviens,
il/elle/on devient,
nous devenons
vous devenez,
ils/elles deviennent
I hope that helps a bit.
-- Chris (not a native speaker).
