In the number series 9, 11, 15, 17, 21, 23, ?, which number should replace the question mark so that the alternating pattern is preserved?

Introduction / Context:
This series uses an alternating pattern where numbers increase in pairs. Many reasoning questions adopt such paired structures to test whether candidates can observe and extend simple but slightly disguised patterns. Identifying the structure of these pairs is the main task here.

Given Data / Assumptions:
- The given series is: 9, 11, 15, 17, 21, 23, ?- Terms appear to be arranged in pairs: (9,11), (15,17), (21,23).- We suspect a consistent rule for pair construction and movement between pairs.

Concept / Approach:
When a series appears to form natural pairs, it is useful to treat each pair as a unit and examine how those pairs are generated. In this series, each pair consists of two numbers that are very close to each other, and there is a fixed gap between the first elements of consecutive pairs. Once the pattern in the first elements is known, we simply add the constant internal difference to get the second element of the next pair.

Step-by-Step Solution:
- Group the terms: (9, 11), (15, 17), (21, 23).- Within each pair, the second number is the first plus 2: 9 + 2 = 11, 15 + 2 = 17, 21 + 2 = 23.- Consider the first numbers of each pair: 9, 15, 21.- Differences between these first terms: 15 - 9 = 6, 21 - 15 = 6.- So the first number of each pair increases by 6.- The next first term should therefore be 21 + 6 = 27.- Hence the next pair is (27, 29), and we only need the first missing number 27 for the series.

Verification / Alternative check:
- Reconstruct the logical pairs: (9, 11), (15, 17), (21, 23), (27, 29).- First elements follow: 9, 15, 21, 27 with constant difference 6.- Second elements follow: first element + 2, which matches every pair.- This confirms that 27 is the missing term in the original series.

Why Other Options Are Wrong:
- 29 and 30 would shift the internal or external differences and break either the pair structure or the 6-step progression.- 28 does not fit naturally into any consistent pattern of first terms increasing by 6 and internal pair difference of 2.- Only 27 preserves both the pairwise difference and the inter-pair movement.

Common Pitfalls:
Many candidates try to treat each term individually rather than as part of a pair and end up searching for a single-step arithmetic or geometric rule. Others might miscalculate the differences or misidentify the pair boundaries. Always look for natural groupings when you see repeated small gaps like +2 appearing regularly.

Final Answer:
The correct number to replace the question mark is 27, so the correct option is 27.