Improve the following sentence, if necessary, by choosing the most appropriate option: "I am waiting for three-quarters of an hour."

Introduction / Context:
This question focuses on the correct tense to describe an action that started in the past and continues up to the present with a specified duration. Tense and preposition combinations with "for" and "since" are a very common test area in competitive English exams. The original sentence uses present continuous in a way that does not match the duration phrase.

Given Data / Assumptions:
- Original sentence: "I am waiting for three-quarters of an hour."
- The speaker started waiting forty five minutes ago and is still waiting now.
- The time expression "for three-quarters of an hour" describes duration up to the present.
- We need to adjust the tense if required, while keeping the meaning unchanged.

Concept / Approach:
As with similar duration expressions, English prefers the present perfect continuous tense when the action started in the past and continues now. The standard pattern is "have been verb ing" plus a duration phrase with "for", for example "I have been waiting for an hour". Present continuous ("I am waiting") describes what is happening at this moment, but when we add "for three-quarters of an hour", the sense is that the waiting began earlier, so present perfect continuous is the natural choice.

Step-by-Step Solution:
Step 1: Notice the duration phrase "for three-quarters of an hour". Step 2: Recognise that this indicates an action in progress from the past until now. Step 3: Select the present perfect continuous form: "have been waiting". Step 4: Combine with the duration phrase to get "I have been waiting for three-quarters of an hour." Step 5: Compare this with the options and confirm that option C matches exactly.

Verification / Alternative check:
Examine option A: "I am waiting since three-quarters of an hour." This is incorrect because "since" should refer to a starting point in time, such as "since 5 o clock", not a period. Option B: "I have waited since three-quarters of an hour." again misuses "since" and feels unnatural. "No improvement" would keep the slightly wrong combination of present continuous with a completed period. Only option C correctly combines "have been waiting" with "for three-quarters of an hour".

Why Other Options Are Wrong:
- "I am waiting since": mixes present continuous with "since", which is not idiomatic for duration in this way.
- "I have waited since": uses present perfect with "since" followed by a period, which is incorrect; "since" should be followed by a point in time.
- "No improvement": fails to fix the tense issue and leaves the sentence sounding non native.

Common Pitfalls:
Candidates often confuse "for" and "since". A simple reminder is: "for" is used with a period of time (for two years, for ten minutes), while "since" is used with a specific starting time (since Monday, since 2010). Also, when mentioning how long something has been happening, think of present perfect continuous rather than present continuous, especially in formal written English.

Final Answer:
The correct improved sentence is I have been waiting for three-quarters of an hour.