In the following sentence on expressing speed, choose the alternative that best improves the underlined part; if no improvement is needed, select "No improvement". He likes to drive his car at a speed of eighty kilometres each hour.

Introduction / Context:
This sentence improvement question focuses on the natural way of expressing speed in English. Motor vehicle speeds are almost always stated using the pattern "kilometres an hour" or "kilometres per hour". The original sentence uses a slightly awkward phrase, and you must select the option that makes the sentence idiomatic and grammatically standard.

Given Data / Assumptions:

• The sentence is "He likes to drive his car at a speed of eighty kilometres each hour."
• The underlined part in the original exam would be "each hour".
• The answer choices given are "every hour", "an hour", "hourly", and "No improvement".
• We are applying standard international usage for speed expressions.

Concept / Approach:
In English, the usual ways of talking about speed are "eighty kilometres an hour" or "eighty kilometres per hour". The phrase "each hour" is grammatically possible but not natural when describing speed. We must choose the option that turns the awkward phrase into a familiar, correct pattern without changing the meaning.

Step-by-Step Solution:
Step 1: Consider the standard collocation. People say "eighty kilometres an hour" when talking about how fast someone drives.Step 2: Replace "each hour" with "an hour": "He likes to drive his car at a speed of eighty kilometres an hour." This sounds natural and correct.Step 3: Test "every hour". This would give "eighty kilometres every hour", which sounds like describing repeated events rather than a continuous speed.Step 4: Test "hourly". Inserting it would produce an awkward phrase "eighty kilometres hourly", which is not standard in conversational or exam English.Step 5: Test "No improvement". Since we have a better and widely accepted alternative, we cannot choose this.Step 6: Therefore, "an hour" is the appropriate improvement.

Verification / Alternative check:
Compare the sentence with common expressions: "The train runs at ninety kilometres an hour", "He was driving at sixty kilometres an hour", and "The limit is fifty kilometres an hour". These all show the same structure. The phrase "each hour" is not normally used with numerical speed; it might occur in other contexts like "he earns so much each hour", but not with speed units. This confirms that "an hour" is a necessary improvement.

Why Other Options Are Wrong:
Option A, "every hour", suggests something happening again and again at hourly intervals, for example "He phones his office every hour". This does not describe continuous driving speed.
Option C, "hourly", usually functions as an adverb or adjective ("hourly wages") and is not combined directly with a number and unit in the same way as "kilometres an hour".
Option D, "No improvement", is incorrect because the phrase "eighty kilometres each hour" is not the standard way of expressing speed.

Common Pitfalls:
Exam candidates sometimes focus only on grammatical correctness and ignore naturalness and collocation. Since "each hour" is not completely wrong grammatically, they might be tempted to keep it. However, sentence improvement questions typically test idiomatic usage, and you should choose the expression that most native speakers would use for that meaning.

Final Answer:
The correct improvement is "eighty kilometres an hour", so the answer is option B.