Which of the following is a pair of co-prime numbers?

1. 8, 45

2. 3, 18

3. 5, 35

4. 6, 39

8, 45: 8 = 2³, 45 = 3² × 5. No common factor other than 1. Co-prime ✓

3, 18: 18 = 2 × 3². Common factor is 3. Not co-prime.

5, 35: 35 = 5 × 7. Common factor is 5. Not co-prime.

6, 39: 6 = 2 × 3, 39 = 3 × 13. Common factor is 3. Not co-prime.

Hence, option 1 is the correct option.