Is this sentence a run on a comma slice or a fragment and how would i re write this the correct way After John finished his prewriting, he began his first draft, he was shocked at how easy it was to write the draft since he had planned out his ideas

The sentence is a run on sentence.

The correct way to re-write this would be:

After John finished his prewriting, he began his first draft. He was shocked at how easily it was to write the first draft since he had planned out his ideas.

Explanation:

the correct answer is:

this is an example of the writer telling through description rather than showing through dialogue.

sometimes writers like to explain explicitely what's happening instead of adding dialogues of the situations., which is the case in this passage,