B is the correct answer to this situation
<span>He was having a fun time with you're uncle at the beach.
The mistake is this sentence uses "you're" which means 'you are'
</span>
<span>He was having a fun time with you are uncle at the beach
makes no sense.
The correct sentence is </span>
<span>He was having a fun time with your uncle at the beach
</span>
