Don't feel bad. Without context, this problem does look strange. Are you sure there is no information before the problem (possibly in the text) saying what elementary (atomic) propositions are? If you are free to come up with your own elementary propositions, then, for what's it worth, you can say

p ∧ (p ∨ ¬p)

where p stands for "every cell of a 9x9 Sudoku puzzle contains at least one number". It would be enough to say just p, but the problem requires a compound proposition, and (p ∨ ¬p) is always true, so it does not change the meaning.