I have got a fox. ... fox is red. a) A

I have got a fox. The fox is red. 

Here we use the article the because it is already similar to us and we know that fox already. We use the indefinite article a first of all when it is countable noun and when we dont know the object.

For example: Give me the pen which is on the table. 

We use the because that pen which is on the table.

Other example: A man asks him a question. 

We use a because we dont know him.