The correct tense for the sentence is option 'd) had been working'.

Explanation:

Past Perfect Continuous tense is used to describe an ongoing action that was happening in the past before another action took place. In this sentence, the action of "working on her assignment" had already started and was in progress when another action, "her friend called," happened. Therefore, we need to use the Past Perfect Continuous tense to show the ongoing action that was happening before the past action.

Here's a breakdown of the sentence:

1. "She (work)": This is the subject of the sentence, referring to a female person.

2. "on her assignment": This phrase tells us what she was working on.

3. "for two hours": This phrase indicates the duration of the action, telling us how long she had been working.

4. "when her friend called": This part of the sentence introduces the second action that occurred in the past.

Now let's analyze the options:

a) "works": This is the simple present tense, which does not match the context of the sentence. It implies a general statement or a habitual action, rather than a specific past event.

b) "worked": This is the simple past tense, which indicates a completed action in the past. However, it does not convey the ongoing nature of the action before her friend called.

c) "has been working": This is the present perfect continuous tense, which is used to describe an ongoing action that started in the past and is still continuing in the present. However, in this sentence, the action of working on her assignment is in the past and not continuing until the present moment.

d) "had been working": This is the past perfect continuous tense, which is the correct choice. It indicates that she was working on her assignment for a duration of two hours before her friend called in the past. The past perfect continuous tense shows the ongoing nature of the action before the past action occurred.

Therefore, the correct tense for the sentence is option 'd) had been working'.