The following sentence has a blank that must be filled with the correct form of the verb. Choose the option that best completes the sentence in standard English. Since last year his condition has been ________.

Introduction:
This fill in the blank question tests your understanding of tense and aspect in English, especially how to express a continuing change that started in the past and is still going on. The phrase "since last year" signals a process that began at a specific time in the past and continues up to the present.

Given Data / Assumptions:
- Incomplete sentence: "Since last year his condition has been ________."
- Options: "deteriorate", "deteriorating", "severely deteriorated", and "deterioratingly".
- We assume the speaker wants to say that his condition has been gradually becoming worse over a period of time.

Concept / Approach:
The structure "has been" is typically followed by a present participle (verb + ing) to form the present perfect continuous tense, when we want to emphasise a continuing action or process. The verb "deteriorate" means to become worse. Therefore, the natural construction is "has been deteriorating", which shows that the worsening of his condition started last year and is still happening.

Step-by-Step Solution:
Step 1: Recognise that "Since last year" is a time expression that often pairs with perfect or perfect continuous tenses.Step 2: Notice the auxiliary verb phrase "has been", which suggests we need a present participle (verb + ing) after it.Step 3: Among the options, "deteriorating" is the present participle form of "deteriorate".Step 4: Substitute it into the sentence: "Since last year his condition has been deteriorating." This sounds correct and expresses ongoing worsening.Step 5: Confirm that the other options do not fit the grammar or the intended meaning as well as "deteriorating".

Verification / Alternative check:
Compare with similar structures: "Since last year he has been working here", "Since childhood she has been suffering from allergies". In each case "has been" is followed by a present participle. Also, the nature of "condition" becoming worse is gradual, so a continuous form is appropriate. "Has been deteriorating" matches this pattern perfectly.

Why Other Options Are Wrong:
Option A, "deteriorate", is the base form of the verb and cannot follow "has been" directly in this structure. Option C, "severely deteriorated", is a past participle phrase and would fit better after "has" alone, as in "has severely deteriorated", but it is not consistent with "has been". Option D, "deterioratingly", is an adverb form that is rarely used and does not complete the verb phrase correctly. Only "deteriorating" functions as the present participle required by "has been".

Common Pitfalls:
Some candidates may try to use "has severely deteriorated" because they are attracted by the strong adverb, but they overlook that the given auxiliary is "has been", not "has". Others may be unfamiliar with the present perfect continuous tense and choose the base form "deteriorate". To avoid such mistakes, pay attention to the exact auxiliary verbs provided in the sentence and recall how they combine with different forms of the main verb.

Final Answer:
The correct completion is "Since last year his condition has been deteriorating."