Which number comes next in the following number series? Carefully observe the pattern in 1, 5, 2, 6, 3, 7, ? and choose the correct alternative from the options.

Introduction / Context:
This question features an alternating number series. The given sequence is 1, 5, 2, 6, 3, 7, ?, and we must find the next term. Such series often consist of two independent subsequences interwoven together. Identifying those subsequences is a key technique in verbal reasoning questions involving numbers.

Given Data / Assumptions:
Series: 1, 5, 2, 6, 3, 7, ?
There is one missing term at the end of the series.
The numbers seem to alternate between two distinct patterns.
The same rule should apply consistently within each subsequence.

Concept / Approach:
We list the terms along with their positions and separate the series into odd and even positions. Often, one subsequence will be increasing slowly (for example, 1, 2, 3, 4, ...) while the other also increases but perhaps from a different starting value. In this series, the first, third, and fifth terms form one subsequence, and the second, fourth, and sixth terms form another.

Step-by-Step Solution:
Write the series with positions: 1st: 1, 2nd: 5, 3rd: 2, 4th: 6, 5th: 3, 6th: 7, 7th: ?. Odd-position terms: 1, 2, 3, ?. Even-position terms: 5, 6, 7. Odd positions clearly follow: 1, 2, 3, 4 (consecutive natural numbers). Even positions follow: 5, 6, 7, 8 (also consecutive natural numbers), although 8 has not yet appeared. The missing term is at the 7th position, which is an odd position. So the next odd-position term after 3 is 4. Therefore, the missing number is 4.

Verification / Alternative check:
Reconstruct the extended series: odd positions as 1, 2, 3, 4 and even positions as 5, 6, 7, 8. Interleaving them gives 1, 5, 2, 6, 3, 7, 4, 8, and so on. This is a completely regular pattern of two increasing sequences interwoven together. The seventh term must therefore be 4, and none of the other options fits this structure.

Why Other Options Are Wrong:
Option A: 6 belongs to the even-position subsequence and has already appeared, so it cannot be the next odd-position term.
Option B: 5 is also part of the even-position subsequence and repeats an earlier value, breaking the smooth pattern.
Option D: 3 has already appeared at the fifth position and does not continue the odd-position sequence 1, 2, 3, 4.
Option E: 8 would be the next even-position term, not the next odd-position term, so it is misplaced here.

Common Pitfalls:
A common mistake is to treat the series as a single progression and search for one complex pattern rather than two simple alternating ones. Another pitfall is to ignore the importance of positions and focus only on the values themselves. In many reasoning questions, carefully separating odd and even positions quickly reveals the underlying rule.

Final Answer:
The number that correctly completes the alternating pattern is 4, so the correct option is 4.