In the following alphabetical letter series, one term is missing. Select the group of letters that correctly completes the sequence: LOP, MPQ, NQR, ORS, ?

Introduction / Context:
This question tests your understanding of simple alphabetical letter series. Such questions are very common in competitive aptitude exams, where you must observe the pattern in each position of the letter groups and extend the sequence correctly. Here, each term is a group of three letters and you have to find the missing term after ORS in the series LOP, MPQ, NQR, ORS, ?.

Given Data / Assumptions:

• Given series of letter groups: LOP, MPQ, NQR, ORS, ?
• Each term consists of three letters in the English alphabet.
• The pattern is assumed to be regular and consistent across all terms.
• We use standard alphabetical positions: A = 1, B = 2, ..., Z = 26.

Concept / Approach:
We focus on each letter position (first, second and third) separately. The main idea is to convert each letter into its numerical position in the alphabet, look for an arithmetic pattern (such as +1 or +2) and apply the same pattern to generate the next term. Once we find the pattern for all three positions, we can construct the missing group of letters and then match it with the given options.

Step-by-Step Solution:
Step 1: Write the first letters of each term: L, M, N, O. Step 2: Convert them to numbers: L = 12, M = 13, N = 14, O = 15. This shows a +1 pattern each time, so the next first letter should be 15 + 1 = 16, which is P. Step 3: Write the second letters: O, P, Q, R. Step 4: Convert them: O = 15, P = 16, Q = 17, R = 18. Again, the pattern is +1, so the next second letter is 18 + 1 = 19, which is S. Step 5: Write the third letters: P, Q, R, S. Step 6: Convert them: P = 16, Q = 17, R = 18, S = 19. The pattern is +1, so the next third letter is 19 + 1 = 20, which is T. Step 7: Combine these results. The missing group of letters is P (first position), S (second position) and T (third position), giving PST.

Verification / Alternative check:
We can quickly verify by writing all five terms with their positions: LOP (12, 15, 16), MPQ (13, 16, 17), NQR (14, 17, 18), ORS (15, 18, 19), PST (16, 19, 20). Each column clearly increases by 1 at every step. This confirms that PST fits the pattern perfectly and is therefore the correct next term in the series.

Why Other Options Are Wrong:
Option A (QTU): Here the first letter Q corresponds to 17 instead of 16, breaking the +1 pattern in the first column. Option B (QST): The first letter Q is again 17, not 16, so the first-letter progression fails. Option D (PTU): The third letter U corresponds to 21, which does not continue the +1 pattern from 19.

Common Pitfalls:
Students sometimes mix up the positions of letters and check only one or two columns instead of all three. Another frequent mistake is to miscalculate letter positions (for example, confusing the numerical positions of letters near the middle of the alphabet). To avoid this, always check each position separately and verify your result against the full pattern of the series.

Final Answer:
The missing term that correctly continues the pattern in all three letter positions is PST, which corresponds to option C.