Final answer:
In the given mathematical relation, the number 3 is related to the number 4, as they form the pair (3, 4).
Step-by-step explanation:
To answer which number 3 is related to in the mathematical relation formed by the pairs (1, 2), (3, 4), and (5, 6), we should look at the second number in the pair that contains the first number 3.
The pair that includes the number 3 is (3, 4). Therefore, in this relation, the number 3 is related to the number 4. So the correct answer would be B) 4.