In this sentence improvement item, choose the correct preposition to replace the bracketed word "(on)" in the sentence about Japan’s feudal overlords finding themselves in financial difficulty.

Introduction / Context:
This question focuses on the correct preposition used with the noun "stress" when describing financial difficulty. The sentence refers to feudal overlords in Japan who were facing financial pressure. The bracketed preposition "on" in the phrase "on financial stress" does not fit standard English usage, so you must select a better alternative that expresses being in a state of difficulty or pressure.

Given Data / Assumptions:

• The sentence describes historical figures facing money related pressure.
• The bracketed phrase is "on financial stress".
• We want to say that they were suffering from this stress.
• Options are "of", "for", "under", and "No improvement".

Concept / Approach:
In idiomatic English, we often use the preposition "under" with nouns like "stress", "pressure", or "attack" to indicate that someone is experiencing them. Expressions such as "under financial stress", "under pressure", and "under strain" are very common. The combination "on financial stress" is not used. "Of" would suggest belonging, and "for" suggests purpose, neither of which fits the context. Therefore, "under financial stress" is the correct phrase to describe their condition.

Step-by-Step Solution:
Step 1: Identify the collocation needed for "financial stress". Step 2: Recall common expressions: "under stress", "under great pressure", "under financial strain". Step 3: Replace "on" with "under" to form "under financial stress". Step 4: Read the sentence again to check for natural flow and correctness. Step 5: Reject prepositions that do not form standard pairs with "stress".

Verification / Alternative check:
The corrected sentence reads: "In the eighteenth century, Japan's feudal overlords, from the shogun to the humblest samurai, found themselves under financial stress." This matches typical historical and economic writing. If you try "of financial stress", it sounds awkward and incomplete. "For financial stress" does not convey the state they were in. Leaving "on" unchanged keeps a non standard phrase. Only "under financial stress" clearly and idiomatically conveys that they were suffering from financial pressure.

Why Other Options Are Wrong:
"Of" usually indicates possession or origin and would require a different sentence pattern.

"For" expresses purpose or benefit, as in "for education", and is not used to describe a state of being stressed.

"No improvement" would keep "on financial stress", which is not a recognised English collocation.

Common Pitfalls:
A frequent error in preposition questions is to focus only on the literal meaning of the word and ignore fixed phrases that native speakers use. A good strategy is to memorise common patterns such as "under stress", "in debt", "at risk", "on duty", and so on. Recognising "under stress" and "under pressure" as fixed expressions makes it easy to select "under" in this question.

Final Answer:
The correct improvement is "under", giving the phrase "under financial stress".