In the following spelling question, four variants of a word are given. You must select the correctly spelt word that matches the standard British English spelling of the verb meaning "to form or express a theory." Which of the following spellings is correct?

Introduction / Context:
This question assesses your spelling knowledge, especially of verbs that are common in academic and formal writing. The word in focus means to form a theory or set of theories about something. In British English, this verb is traditionally spelt with s rather than z. Spelling questions like this often appear in exams because they check attention to detail and familiarity with standard written English.

Given Data / Assumptions:

• You are asked to choose the correct spelling of the verb meaning to form or express a theory.
• The options are theurise, theoris, theuris and theorise.
• The root of the word is theory, and the verb is formed by adding ise in British English.
• You are expected to recognise both the correct vowel sequence and the appropriate ending.

Concept / Approach:
The verb derived from theory is theorise in British spelling and theorize in American spelling. The long vowel sound comes from the eo sequence in theory, which is preserved in the verb. The correct formation drops the final y from theory and adds ise, giving theorise. Any change to this internal vowel combination or incorrect ending will lead to a misspelt form. Therefore, the right answer must retain the segment theor and use the standard suffix ise.

Step-by-Step Solution:
Step 1: Start from the noun theory, spelt t h e o r y. Step 2: To form the verb, remove the y and add ise in British English, resulting in t h e o r i s e. Step 3: This gives the spelling theorise, with the key segment theor plus ise. Step 4: Compare with option D, theorise, which exactly matches this pattern. Step 5: Observe that options A, B and C alter either the vowel sequence (eur instead of eor) or the ending (missing e at the end), and thus do not match the correct word. Therefore, option D is correct.

Verification / Alternative check:
You can check related forms to confirm the spelling. The noun is theorist, not theurist, and the adjective is theoretical, not theuretical. These related words keep the e o pattern from theory. It is logical that the verb should also maintain this pattern. Another simple test is to pronounce the word slowly and imagine writing each syllable: the o rise. This guides you toward theeo and rise sequence, which appears correctly in theorise.

Why Other Options Are Wrong:
Theurise and theuris replace eo with eu, which is not supported by the original noun theory and therefore produces a nonstandard spelling. Theoris omits the final e, which is necessary in the standard spelling of the verb. None of these alternatives appear in reputable dictionaries. Only theorise preserves the correct root and adds the expected verb ending for British English usage.

Common Pitfalls:
Some candidates may confuse British and American spellings, expecting theorize as the only correct form. Exams that focus on British spelling generally prefer theorise instead. Others may misremember the internal vowels and attempt to approximate the sound with eu instead of eo. To avoid these mistakes, it is useful to learn word families like theory, theoretical, theorist and theorise together, reinforcing both meaning and spelling across related forms.

Final Answer:
The correctly spelt British English verb meaning to form or express a theory is theorise.