A single letter series is given as I, V, ?, L. Treating these letters as Roman numerals, which letter should fill the blank to best complete the series?

Introduction / Context:
This series question uses letters that are also Roman numerals. The given sequence is I, V, ?, L. The task is to recognise that the pattern is based on increasing Roman numeral values and then identify the missing Roman numeral letter that fits naturally between V and L in that progression. This tests awareness of both the Roman numeral system and simple series completion.

Given Data / Assumptions:

• Given sequence: I, V, ?, L • Roman numeral values: I for 1, V for 5, X for 10, L for 50, C for 100, D for 500, M for 1000. • Options: C, D, M, X • Assumption: The series follows the standard ascending order of Roman numeral symbols.

Concept / Approach:
First we confirm that all given letters are valid Roman numeral symbols. Next, we recall their standard increasing order used to represent numbers. In the usual Roman numeral sequence, I comes before V, then X, then L, then C, and so on. Once we place the given letters into this familiar order, we can directly read off which letter must appear between V and L to preserve the natural Roman numeral progression.

Step-by-Step Solution:
Step 1: Note that I, V, and L are all valid Roman numeral symbols. They represent 1, 5, and 50 respectively. Step 2: Recall the standard Roman numeral sequence: I, V, X, L, C, D, M. Step 3: Place the given sequence into this order. Starting from I we have I, then V, then next should be X, and later comes L. Step 4: Therefore the missing symbol between V and L in the Roman numeral order is X, which represents 10. Step 5: Among the options C, D, M, and X, only X fits this position in the standard sequence.

Verification / Alternative check:
We can also check by values: I equals 1, V equals 5, X equals 10, and L equals 50. The given series can then be viewed as 1, 5, ?, 50. Looking at the set of basic Roman numerals, the next base value after 5 is 10, represented by X. Values 100, 500, and 1000, represented by C, D, and M, are far beyond 50 and therefore would not sit between V and L in a logical ascending list. This numeric perspective confirms that X is the only appropriate missing symbol.

Why Other Options Are Wrong:
C represents 100, which comes after L (50), not between V (5) and L (50), so it cannot be the missing term. D represents 500 and M represents 1000, both much larger than 50 and unsuitable for the gap immediately after V in a basic Roman numeral progression.

Common Pitfalls:
Some candidates may forget the exact order of Roman numeral symbols and try to guess based on approximate numeric size. Others may not immediately recognise that the letters are being used as numerals rather than regular alphabet positions. Remembering the standard sequence I, V, X, L, C, D, M and writing it down can quickly resolve confusion in similar questions.

Final Answer:
The letter that completes the Roman numeral series I, V, ?, L is X.