In this English grammar improvement question, choose the alternative that best improves the bracketed part of the sentence: "The base of the cooking vessels would have a thick coating of carbon deposit and (clean) it was a pain."

Introduction / Context:
This question checks your understanding of correct verb forms and sentence structure in English. The sentence describes a habitual situation where the base of cooking vessels has a thick layer of carbon, and the action of cleaning it is difficult. You need to decide which form of the verb will make the sentence grammatically correct and natural in standard English usage.

Given Data / Assumptions:

• The original sentence is: "The base of the cooking vessels would have a thick coating of carbon deposit and (clean) it was a pain."
• You must choose the correct form of "clean" to complete the sentence.
• The sentence describes cleaning as an activity that is difficult or painful.

Concept / Approach:
When we talk about an activity in general, especially as the subject of a clause, we often use the gerund form of a verb, that is, the verb plus "-ing". In the structure "and cleaning it was a pain", "cleaning" functions as a noun, describing the action. Using "cleaned" or "cleans" here would break the structure and lead to incorrect grammar. Therefore, understanding the function of gerunds is key to solving this question correctly.

Step-by-Step Solution:
Step 1: Identify what role the word after "and" must play. It should name an activity that is difficult. Step 2: Recognise that the clause "cleaning it was a pain" needs a gerund "cleaning" as the subject of the verb "was". Step 3: Option C "cleaning" gives exactly this gerund form. Step 4: Option A "cleaned" would create "and cleaned it was a pain", which is ungrammatical because "cleaned" is a past tense or past participle, not a subject. Step 5: Option B "cleans" is present simple and cannot function logically in that position. Step 6: "No improvement" is wrong because the bracketed base form "clean" is not correct in the given structure. Step 7: Option E "to clean" also changes the structure and sounds less natural than the gerund in this parallel construction.

Verification / Alternative check:
A good way to verify is to focus on the part after "and" only. Say "Cleaning it was a pain" aloud. It sounds natural and grammatically correct. Try replacing "cleaning" with "cleaned" or "cleans" and you will notice that the sentence becomes awkward and incorrect. Therefore, the gerund form is the most appropriate and standard choice in this context.

Why Other Options Are Wrong:
"Cleaned" would normally need a subject and an object, for example, "I cleaned it", so it does not work as the subject of "was". "Cleans" is simple present tense and again cannot be used as the subject in this way. "No improvement" is incorrect because the base form "clean" does not fit the structure. "To clean" introduces an infinitive clause, which would make the sentence "and to clean it was a pain". This is possible but less natural and does not match the parallel style of the sentence as well as "cleaning".

Common Pitfalls:
Students often ignore whether the verb is acting as a noun or as part of a tense. Gerunds are frequently tested in competitive exams because many sentences require verb forms that function as subjects or objects. To avoid mistakes, always check whether the word is naming an activity or showing an action in time. If it names an activity, the "-ing" form is usually required.

Final Answer:
The correct improvement is cleaning.