今年の冬は去年___寒くないです。
Q. Which particle completes 'not as cold as last year'?
Translation
This winter is not as cold as last year.
Explanation
'Not as ... as X' is X + ほど + negative: 去年ほど寒くない. より appears in affirmative comparisons (去年より寒い 'colder than last year'); with a negative, ほど is the set pattern.