Each question names a number and asks whether it is prime or composite. A number is prime when it has exactly two factors - one and itself - and composite when it has more. Deciding means looking for a factor, not recalling a list.
The numbers are always at least two digits. Below ten the answer is memorised rather than worked out, and the skill here is knowing what to divide by: check 2, 3, 5, 7 and stop once the divisor squared passes the number.
A whole number with exactly two factors: one and itself. 29 is prime because nothing between 2 and 28 divides it. 51 is not, because 3 × 17 is 51.
Neither. It has only one factor, so it fails the definition of prime, and it has no others, so it is not composite. It does not appear on these sheets.
Try 2, 3, 5, 7, 11 and so on, and stop when the divisor multiplied by itself passes the number. For 91 you only need to reach 7 - and 7 × 13 is 91, so it is composite.
Fourth and fifth grade, taught alongside factors - which is exactly what a primality check is, so both are on the same generator here.