In the following question, several spellings of the same word are given. Select the correctly spelt word that means to develop or form a theory.

Introduction / Context:
This spelling question focuses on a verb that is frequently used in academic writing, philosophy, and science. The word means to develop or form a theory about something. Many learners see both British and American spellings, which can add confusion. Knowing the standard forms and their variants is important for exams, where any deviation from accepted spelling is treated as an error.

Given Data / Assumptions:
The stem asks for the correctly spelt form of a known verb.

The intended meaning is to form a theory.
In British English, the spelling generally uses ise, as in theorise.
In American English, the spelling generally uses ize, as in theorize.
We assume the test follows British conventions, as many competitive exams do.

Concept / Approach:
The root word is theory, which ends with y. When forming the verb in British English, the y changes to i and the ending ise is added, giving theorise. In American English, the ending ize is used, giving theorize, which is also a valid spelling but may not be the preferred one in this exam context. The other options either change the vowels incorrectly or drop letters. A structured approach is to recall the base noun theory, then apply the standard verb forming rule and compare the result with the options.

Step-by-Step Solution:

Verification / Alternative check:
We can check by forming related words like organise, criticise, and generalise, which also use the ise ending in British English. The structure is consistent across many verbs. If the exam allowed American spelling, theorize would also be acceptable, but the presence of theorise in the options and the exam tradition suggests that theorise is preferred. Dictionaries list theorise as the British form meaning to develop an explanation based on observation and reasoning.

Why Other Options Are Wrong:
Theurise and theuris incorrectly replace the o with u, breaking the link with the root theory and creating a non standard form. Theoris drops the final e and does not clearly indicate verb formation, looking incomplete and unnatural in English orthography. Theorize is a correct American spelling but does not match the typical British English standard that most such exams expect, and the question asks for one correct spelling, making theorise the safest and most accurate choice in this context.

Common Pitfalls:
One common pitfall is confusing British and American variants and assuming that any variant is acceptable in all exams. Another issue is ignoring the root word and guessing based on sound alone, which leads to errors such as replacing vowels arbitrarily. Learners should connect theorise directly to theory and remember the simple y to i rule when adding ise in British English. Doing so helps secure marks not only in spelling questions but also in writing tasks where correct form signals precision and care.

Final Answer:
The correctly spelt word, following standard British English, is theorise.