That would be a fragment. Its not a complete sentence
What sentence I’m confused
The correct answer is The following Saturday, I was walking by the dirt lot again, and there was Ben digging up the dirt with a shovel.
Explanation:
In this passage, the narrator describes experiences related to the creation of a community garden. In sentence 5, the author describes what seems to be the beginning of community garden "I was walking by the dirt lot again, and there was Ben digging up..."; however, this sentence does not show the relationship between this event and the events previously described.
In this context, it is necessary to add a time relationship or at least a connector that shows how Been acting strangely led to him began working to create a garden. According to this, the best option is "The following Saturday, I was walking by the dirt..." because this clearly shows this last event occurred after Ben decided the dirt lot was a perfect place to create a community garden.