The phrase "sole responsibility" means that someone or something is entirely accountable or in charge of a particular task or duty. They are the only one responsible and no one else shares the responsibility with them. Full definition
For instance if you are no longer responsible for making mortgage payments or if you have taken on sole responsibility of the mortgage payments, your insurance policy should reflect this change. (insuranceindepth.com)
The hormone that bears sole responsibility for the contraction of the uterus is oxytocin. (intelligentmother.com)
Research shows that one in four wives and girlfriends take sole responsibility for choosing the next holiday. (thetravelmagazine.net)