Explanation:

- Option 1: "will be send to"

- This is incorrect. It should be "will be sent to" because "send" should be in its past participle form here.

- Option 2: "are sending to"

- This changes the verb tense to present continuous, which doesn't fit the requirement for instructions about applications.

- Option 3: "No improvement"

- The original sentence "Applications are to be sent to the principal before 30th May" is correct as is. The passive construction is appropriate for the context.

- Option 4: "were being send to"

- This is incorrect. It should be "were being sent to" to be grammatically correct, and past continuous is not needed here.

Option 3: No improvement