Explanation:

Sure, let's break down the sentence and the options:

- Option 1: We must get

This segment is grammatically correct. "We must get" is a correct phrase indicating an obligation.

- Option 2: this post

This segment is also grammatically correct. "This post" identifies the object to be sent.

- Option 3: soon as possible

This commonly used phrase should be "as soon as possible." It is missing the initial "as."

- Option 4: send as

The error is here. The correct form should be "sent." "Send" should be in the past participle form to correctly follow "get."

Correct Answer:

- Option 4: send as