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

1. 19, 21

2. 43, 47

3. 59, 61

4. 73, 79

Twin primes are pairs of primes whose difference is 2.

19, 21: 21 = 3 × 7 is not prime.

43, 47: difference is 4, not twin primes.

59, 61: both are primes and difference is 2. ✓

73, 79: difference is 6, not twin primes.

Hence, option 3 is the correct option.