To create a horizontal rule in HTML, we can use the <hr> tag.

Now let's go through each option and determine if it is the correct answer:

A. <b>: This tag is used to make text bold, not to create a horizontal rule. So, this option is incorrect.

B. </b><p>: This tag is used to create a paragraph, not a horizontal rule. Therefore, this option is incorrect.

C. <br>: This tag is used to insert a line break, not a horizontal rule. Hence, this option is incorrect.

D. </p><hr>: This tag is specifically used to create a horizontal rule. So, this option is the correct answer.

Therefore, the correct answer is D - <hr>.