Question

(6) In the summer, school started at 6 a.m. (7) School did not end until 5 p.m. Which is the most effective way to combine sentences (6) and (7)?

The summer school day, ending at 5 p.m., started at 6 a.m.

In the summer, school started at 6 a.m. and did not end until 5 p.m.

Starting at 6 a.m. and not ending until 5 p.m. was school in the summer.

Not ending until 5 p.m., school in the summer started at 6 a.m.

Answer 1

Number 2 is correct

Answer 2

The first one :)
In the second one, your verb 'was' is misplaced, and makes it sound a bit off.
In the 3rd one, 'in the summer' is misplaced, and the beginning of your sentence feels a bit weird.
You should look for the common ground in the sentences, namely "school", and then just start with the first one, and adding the second one while leaving out the common ground element and adapting a few minor things.