In the sentence "From the last five days it has been raining torrentially.", the underlined phrase needs to be checked. Select the option that best improves this time expression, or choose No improvement if the original is correct.

Introduction / Context:
This sentence improvement question focuses on the correct use of prepositions with time expressions, especially when describing a situation that started in the past and continues up to the present. The sentence uses the present perfect continuous tense has been raining torrentially, which normally pairs with specific time expressions like for the last five days or since Monday. The task is to select the preposition that forms an idiomatic phrase with the last five days.

Given Data / Assumptions:

• Sentence: From the last five days it has been raining torrentially.
• The underlined part is From the last five days.
• Options: Since the last, for the last, Since last, No improvement.
• The intended meaning is that the heavy rain started five days ago and has continued up to now.

Concept / Approach:
In English, for is used with periods of time to express duration, as in for three hours, for two weeks, for many years. Since is used with a point in time, such as since Monday or since 2010. The phrase the last five days refers to a duration, not a specific starting date, so it should be introduced by for, not since or from. Therefore, the correct expression in the context of the present perfect continuous tense is for the last five days.

Step-by-Step Solution:
Step 1: Identify the tense used in the main clause: has been raining torrentially, which is present perfect continuous. Step 2: Recall that this tense is normally used with either since plus a starting point or for plus a length of time. Step 3: Recognise that the last five days describes a duration of time rather than a specific starting date, so it should be paired with for. Step 4: Evaluate option B, for the last, which produces for the last five days it has been raining torrentially. This is a natural and correct English sentence. Step 5: Evaluate option A, Since the last, which would give Since the last five days it has been raining, an unidiomatic and incorrect expression because since does not normally take such a phrase. Step 6: Evaluate option C, Since last, which is incomplete without a specific time unit such as Monday or week. Since last five days is not grammatical. Step 7: Reject No improvement because From the last five days is not standard usage for duration in this tense.

Verification / Alternative check:
Compare with other correct examples: It has been snowing for the last two hours, She has been studying for the last three months, They have been living here for the last ten years. All of these use for the last plus a period of time. Using from or since with this structure would sound incorrect to a native speaker. The sentence in the question follows exactly the same pattern, so for the last five days is the only acceptable choice.

Why Other Options Are Wrong:
Since the last five days mixes the point in time use of since with a period of time, which is not how English marks duration. Since last by itself is incomplete and would need a noun such as year or week to make sense. No improvement would preserve the unidiomatic from the last five days, which should be changed to for the last five days.

Common Pitfalls:
Learners frequently confuse for and since because both are used with perfect tenses. A reliable rule is that for goes with lengths of time and since goes with starting points. When you see phrases like the last three days or many years, you should think of them as durations and choose for accordingly. Practising with many examples will help fix this difference in your mind and improve your accuracy in error spotting and sentence improvement questions.

Final Answer:
The correct improvement is for the last, giving: For the last five days it has been raining torrentially.