I choose GROUPING, and still think it is the correct answer.
As others already said COUNT only doesn't ignore NULLs when used with the *. But * is not a value, so it can't be NULL.
I think 1 + 2 + 3 + NULL is the same as 1 + 2 + 3 + 0.
In relation to this, I don't fully agree. For me 1 + 2 + 3 + NULL should return NULL, the same as that 'string1' + 'string2' + 'string3' + NULL returns NULL.