Fill in the blank with the correct word or phrase.
Do you know it will be cold tonight?​

This question is missing the options. I've found the complete question online. It is the following:

Fill in the blank with the correct word or  phrase.

Do you know it will be cold tonight?

A. for

B. and

C. is

D. if

The correct way to complete the sentence is by using letter D. if:

Do you know if it will be cold tonight?

Explanation:

The sentence we are analyzing here is an example of indirect question. When we form an indirect question from what was originally a yes/no question, we need the help of the word "if". The original question in this case was "Will it be cold tonight?" As we add "do you know", we also need to add "if":

- Do you know if it will be cold tonight?

Another example just to better illustrate this is:

- Is she a doctor?

- Do you know if she is a doctor?