posted by Lily .
(upside down question mark)De quien es _____ sueter?
The noun 'sueter' (which should have an accent above the first e) means sweater and is masculine. Therefore the answer is (a) este.
If you used "su", you would already know the answer to the asked question, which makes no sense.
the correct answer is A because sueter is masculine and singular; esta is used for feminine singular
Put it all together correctly and you will have: ¿De quién es este suéter? = Whose sweater is this?