Explanation:

- Sentence Analysis: The sentence talks about the requirement for sending applications to the Principal by a specific date.

- No improvement: "Applications are to be sent to" is correctly structured. It is in the passive voice, indicating that applications must be sent by a certain deadline.

- Option 1 (will be send to): Incorrect tense. Should be "will be sent" for future passive voice.

- Option 2 (are sending to): Incorrect form. It suggests an active continuous action, not fitting the requirement of the sentence.

- Option 4 (were being send to): Incorrect tense and form. Should be "were being sent" for passive past continuous, not appropriate here.

- Correct Answer: Option 3 - No improvement