Question

It was a lovely sunny morning. A duck at a farm sat on her nest.................a young one stepped out

Answer 1

Thanks for your question !

It was a lovely sunny morning. A duck at a farm sat on her nest and a young one stepped out.

Here I used 'and' because it suits to be there. We can also join the sentence by using 'then'.

It is more grammatically correct so I used 'and'.