I have got a fox. ... fox is red. a) A b)The c)-

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 don’t 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 don’t know him.