The correct answer is (c) WALKER.
In the code, each letter of the word “SUMMER” is replaced by the letter that is three places after it in the alphabet. So, S becomes R, U becomes N, M becomes E, M becomes R, E becomes N, and R becomes R.
Following this pattern, the word “WINTER” would be coded as “WALKER”.
The other options are incorrect because they do not follow the pattern of the code. For example, in option (a), SUITER, the letter S is replaced by the letter U, which is only two places after it in the alphabet. In option (b), VIOUER, the letter U is replaced by the letter V, which is four places after it in the alphabet. In option (d), SUFFER, the letter S is replaced by the letter F, which is six places after it in the alphabet.