To answer this question, you need to understand the concept of verb tenses.
The correct answer is D) have been working.
Option A) was working - This option is incorrect because it refers to a past action that is not ongoing.
Option B) have worked - This option is incorrect because it refers to a past action that is not ongoing. It does not explain why the hands are currently dirty.
Option C) worked - This option is incorrect because it refers to a completed past action and does not explain why the hands are currently dirty.
Option D) have been working - This option is correct because it uses the present perfect continuous tense, which indicates an action that started in the past and is still ongoing. "I have been working in the garden" implies that the person has been working in the garden for some time, and as a result, their hands are dirty.
Therefore, the correct answer is D) have been working.