Answer: Option B
The given sentence is in the present perfect continuous tense, which is used to describe an action that started in the past and is still continuing in the present. Therefore, the most appropriate option to replace the italicized phrase would be (B) "have been attending."
Let's understand why the other options are not correct:
A. "Were attending" is in the past continuous tense, which implies that the action has already finished. However, the original sentence suggests that the students are still attending the class.
C. "Have attended" is in the present perfect tense, which indicates that the action started in the past and is now complete. However, the sentence conveys that the students are still attending the class.
D. "To have attended" is in the infinitive form, which is not suitable for describing an action that is currently happening.
E. "No correction" is not the correct option as there is an error in the given sentence.
Now, let's understand the usage of the present perfect continuous tense and its formation:

• The present perfect continuous tense is used to describe an action that started in the past and is still continuing in the present.
• The formation of the present perfect continuous tense is: subject + have/has been + present participle (-ing form) of the main verb.
• Example: I have been studying English for three years.

In conclusion, the correct option to replace the italicized phrase in the given sentence is (B) "have been attending," which accurately conveys that the students started attending the class on 6th February and are still attending it at present.