In the following question, four number pairs are given. In each pair, the number on the right side is related to the number on the left side by a simple addition rule. Three pairs show the same increase of 1, while one pair shows a different increase. Select the odd number pair from the given alternatives.

Introduction / Context:
This is a basic number reasoning question involving simple differences between pairs of numbers. You are given several number pairs where each right hand number is obtained from the left hand number using a rule. In such odd one out questions, most pairs follow a common rule and one pair breaks it. Recognising this difference is important for mastering number relationships and series questions in aptitude tests.

Given Data / Assumptions:
- The options are 11 – 12, 13 – 14, 17 – 18, 19 – 21, and 21 – 22.
- In most pairs, the right number is obtained by adding 1 to the left number.
- Exactly one pair does not follow the pattern of a difference equal to 1 between the two numbers.

Concept / Approach:
The approach is straightforward. For each pair, subtract the left number from the right number and note the difference. If most pairs have a difference of 1 and one pair has a different difference, that different pair will be the odd one out. This type of pattern is common in elementary number series and can be solved very quickly once you are comfortable with simple subtraction.

Step-by-Step Solution:
Step 1: For 11 – 12, difference is 12 - 11 = 1.Step 2: For 13 – 14, difference is 14 - 13 = 1.Step 3: For 17 – 18, difference is 18 - 17 = 1.Step 4: For 21 – 22, difference is 22 - 21 = 1.Step 5: For 19 – 21, difference is 21 - 19 = 2, which is not equal to 1.Step 6: Since only the pair 19 – 21 shows an increase of 2 rather than 1, it breaks the common rule and is the odd one out.

Verification / Alternative check:
You can also look at these as mini sequences of consecutive integers. 11, 12 and 13, 14 and 17, 18 and 21, 22 are all consecutive numbers. However, 19, 21 are not consecutive because 20 is missing between them. This gap of 2 confirms that 19 – 21 behaves differently from the other pairs, which all maintain a gap of 1.

Why Other Options Are Wrong:
The pairs 11 – 12, 13 – 14, 17 – 18, and 21 – 22 all share the property that the second number is exactly one more than the first number. They represent successive integers with no number skipped in between. Since they follow the same simple rule, these four pairs form the main pattern and therefore cannot be selected as the odd pair.

Common Pitfalls:
Because the numbers are small and close to each other, some students may read quickly and overlook the two step jump in 19 – 21. Another potential mistake is to search for complex multiplication or factor relationships when a simple difference check is sufficient. In exam conditions, always start with the simplest possible pattern such as addition or subtraction before considering more complex rules.

Final Answer:
19 – 21