The series lists number pairs: (1, 3), (4, 6), (7, 9), (?, 12). To find the missing number, notice that within each pair, the second number is always exactly 2 more than the first: 3 minus 1 = 2, 6 minus 4 = 2, 9 minus 7 = 2. This constant gap of 2 should also hold for the last pair. Let's check each option against this rule, and cross-verify using the pattern formed by the first numbers of each pair (1, 4, 7, ...), which increases by 3 each time.
Only the value 10 satisfies both the constant within-pair gap of 2 and the steady +3 growth of the first numbers across pairs.
Therefore, the correct answer is 10.
The sequence is written as four pairs: (1, 3), (4, 6), (7, 9), (?, 12). Instead of checking gaps by eye, we can build an explicit formula for the first number of the n-th pair and test it against every option. Numbering the pairs n = 1, 2, 3, 4, the first numbers so far are 1, 4, 7, an arithmetic progression with a common difference of 3 and a first term of 1, giving the general formula \( a_n = 3n - 2 \).
The explicit formula \( a_n = 3n - 2 \), built directly from the first three pairs, predicts exactly one value for the missing term.
Therefore, the correct answer is 10.