Answer:
Option D, 7, is the right answer.
Explanation:
It is given that 28 slips of papers are numbered through 1 to 28.
Accordingly, there are 28 numbers.
For a fair decision, the number of slips with each one must be the same.
Therefore, the decision will only be fair if the number of people in the group is a factor of 28.
The factors of 28 are - 1, 2, 4, 7, 14 and 28.
In this way, 7 is the only factor of 28 among the given options.
Hence, the answer will be 7.