A part of the sentence is underlined. Choose the alternative that improves the underlined part; if no improvement is needed, select "No improvement": The trade union is hell bent at getting its demands fulfilled.

Introduction / Context:
This question checks familiarity with a common English expression and the correct preposition that follows it. Sentence improvement items often focus on such fixed combinations because they reveal whether a candidate has read enough authentic English to recognise natural patterns. The phrase hell bent is an idiomatic adjective phrase that takes a specific preposition when followed by a gerund or noun.

Given Data / Assumptions:
The sentence is: The trade union is hell bent at getting its demands fulfilled. The underlined words are hell bent at, and the answer choices provide several prepositions, including in, about, on, and so on. The meaning is that the trade union is very determined to have its demands accepted, and the structure must show strong determination plus the goal.

Concept / Approach:
In standard English, the fixed expression is hell bent on something, meaning extremely determined or stubbornly committed to achieving a particular aim. When this expression is followed by a gerund or noun phrase, on is the preposition normally used. The combinations hell bent at, hell bent in, or hell bent about are not idiomatic and sound wrong to a native speaker. Therefore, to make the sentence both grammatical and natural, we must replace at with on.

Step-by-Step Solution:
Step 1: Identify the descriptive phrase hell bent, which expresses strong determination.
Step 2: Recall the standard collocation hell bent on doing something.
Step 3: Notice that the sentence currently uses hell bent at, which does not match the known pattern.
Step 4: Replace at with on to produce hell bent on getting its demands fulfilled.
Step 5: Read the full sentence to confirm that it now sounds natural and correct.

Verification / Alternative check:
Consider similar sentences: He is hell bent on winning the match, or They are hell bent on destroying the evidence. In each case, on introduces the goal of the determination. If we substitute at, the phrases feel wrong and unnatural. Applying this pattern back to the original exam sentence confirms that hell bent on is the expected form and that the correction is valid.

Why Other Options Are Wrong:
Option A, hell bent in, is not a recognised collocation in English and does not link naturally with getting its demands fulfilled. Option B, hell bent about, expresses a vague feeling rather than focused determination on a task. Option D, No improvement, would keep the incorrect at and therefore is not acceptable. Option E, hell bent for, is occasionally seen in older or dialectal usage but is not the standard modern form and is not the answer expected in a competitive examination setting.

Common Pitfalls:
Students sometimes treat prepositions as interchangeable, assuming that any small word after an adjective will work. However, many English expressions are fixed combinations that must be learned by exposure and practice. Memorising short chunks like interested in, good at, capable of, and hell bent on is much more useful than memorising isolated preposition rules. Using such chunks in your own sentences will make the patterns feel natural and reduce errors in exams.

Final Answer:
The correct improvement is hell bent on, so the sentence should read: The trade union is hell bent on getting its demands fulfilled.