Select the most appropriate option to improve the bold part of the sentence.
This session is an improvement rather the previous one.

Correct option is D

The correct substitution of the highlighted words is (d) over.
Explanation: The idiomatic collocation is “an improvement over/on/upon [something].” The preposition “rather” is incorrect here; we need a preposition that links “improvement” to the benchmark (“the previous one”). Hence, “This session is an improvement over the previous one.”
Grammatical rule used: After the noun improvement, use:
· improvement in + field/quality/metric (e.g., improvement in accuracy), and
· improvement over/on/upon + a precedent/comparison target (e.g., over the previous model).
· Example: “Her score shows a clear improvement over last year’s performance.”
Information booster:
· improvement in = quality/measure: improvement in health/speed/clarity
· improvement over/on = comparison with another instance: improvement over last attempt