Choose the correct sentence: With the vial set inside the fly box, all the flies could be put to sleep within seconds With the vial settled inside the fly box, all the flies could be put to sleep within seconds

With the vial set inside the fly box, all the fly could be put to sleep in seconds
With the vial settled inside the fly box, all the flies could be put to sleep within seconds
With the vial set inside the fly box, all the flies could be putting to sleep within seconds
With the vial set inside the fly box, all the flies could be put to sleep within seconds

Answer: With the vial settled inside the fly box, all the flies could be put to sleep within seconds

With the vial set inside the fly box, all the flies could be put to sleep within seconds

"Set" is the correct past participle (not "settled") for passive construction.
"Flies" is the correct plural of "fly."
"Could be put to sleep" is the correct passive form.
"Within seconds" is correct and natural English.