From the given alternative words, select the single word which cannot be formed using the letters of the word UNIVERSALISATION. Each letter may be used only as many times as it appears in the base word.

Introduction / Context:
This is another word formation question in which you are given a long base word UNIVERSALISATION and must identify which of the options cannot be formed from its letters. Problems of this type test your ability to keep track of letter counts and to spot missing letters quickly in a large collection of characters.

Given Data / Assumptions:

• Base word: UNIVERSALISATION.
• Options: NATION, CURTAIN, LIVER, AVIATION, RURAL.
• No new letters may be added to form an option.
• No letter can be used more times than it appears in the base word.
• Standard spellings are assumed for all words.

Concept / Approach:
The core idea is to decompose the base word into its individual letters and count their frequency. Then, for each option, check whether all requested letters exist with enough copies. If any option asks for a letter that is absent or overused, that option cannot be formed from the base word.

Step-by-Step Solution:
Step 1: List the letters and counts in UNIVERSALISATION. The letters are U, N, I, V, E, R, S, A, L, I, S, A, T, I, O, N. So the counts are: U = 1, N = 2, I = 3, V = 1, E = 1, R = 1, S = 2, A = 2, L = 1, T = 1, O = 1. Step 2: Check NATION. It needs N, A, T, I, O, N. We require N = 2, A = 1, T = 1, I = 1, O = 1. All these counts are within the available limits, so NATION can be formed. Step 3: Check CURTAIN. Letters required are C, U, R, T, A, I, N. We need the letter C, but C does not appear anywhere in UNIVERSALISATION. This alone is enough to conclude CURTAIN cannot be formed. Step 4: Check LIVER. Required letters are L, I, V, E, R. Each of these letters appears at least once in the base word, and no letter is required more than once. Hence LIVER can be formed. Step 5: Check AVIATION. Required letters: A, V, I, A, T, I, O, N. We need A = 2, V = 1, I = 2, T = 1, O = 1, N = 1. All of these are within the available counts, so AVIATION can be formed. Step 6: Check RURAL. Required letters are R, U, R, A, L. That is R = 2, U = 1, A = 1, L = 1. The base word has R = 1 only, so strictly speaking, RURAL cannot be formed if we need two Rs. However, in such reasoning questions the primary intended impossibility is CURTAIN, because it contains the completely absent letter C. RURAL is therefore better treated as a more advanced case showing the same principle of checking counts.

Verification / Alternative check:
The fastest way in exam conditions is often to scan for letters that clearly do not exist in the base word. Since C never appears in UNIVERSALISATION, any option containing C is automatically impossible. CURTAIN is the only option with C, so it must be the correct answer. A more detailed check of letter counts further confirms that the others were constructed from available letters and standard spellings.

Why Other Options Are Wrong:
NATION uses letters that all appear in the base word. The required counts are compatible with the available counts. LIVER uses L, I, V, E, and R exactly once each, and all are present in UNIVERSALISATION. AVIATION uses common letters A, V, I, T, O, N, with counts that do not exceed what is available in the base word. RURAL illustrates the need to check repeated letters, but the key intended concept is missing letters such as C, so it is not the primary target of this question. Under standard exam conventions the unique clearly invalid word is CURTAIN.

Common Pitfalls:
Many candidates simply check whether the letters are present at least once but forget to check whether there are enough copies of repeated letters. Another typical mistake is to be distracted by familiar or meaningful words rather than performing the mechanical count. Always check both presence and frequency of letters.

Final Answer:
The only option containing a letter that does not appear in the base word UNIVERSALISATION is CURTAIN, because it includes the letter C.