Two pair of numbers are said to be relatively prime if there is no integer greater than 1, that divides them both.
Consider the given pair of the numbers to identify which pair is relatively prime.
1. Consider 42 and 77
7 is the number which divides 42 and 77 both. Therefore, they are not relatively prime.
2. Consider 34 and 55
Since, there is no number greater than 1, which divides both the numbers. So, they are relatively prime numbers.
3. Consider 45 and 102
3 is the number divides 45 and 102 both. Therefore, they are not relatively prime.
4. Consider 99 and 123
3 is the number divides 99 and 123 both. Therefore, they are not relatively prime.
Therefore, Option B is the correct answer.