Answer Description

The only ports that the iPhone 6S and iPad Pro have for interacting with a computer are Lightning ports, thus USB to Lightning cable is the correct answer.

Wikipedia

Lightning is a proprietary computer bus and power connector, created and designed by Apple Inc. It was introduced on September 12, 2012 (2012-09-12), in conjunction with the iPhone 5, to replace its predecessor, the 30-pin dock connector. The Lightning connector is used to connect Apple mobile devices like iPhones, iPads, and iPods to host computers, external monitors, cameras, USB battery chargers, and other peripherals. Using 8 pins instead of 30, Lightning is much smaller than its predecessor. The Lightning connector is reversible. The plug is indented on each side to match up with corresponding points inside the receptacle to retain the connection.In 2018, Apple began transitioning to USB-C on iPad Pros and accessories. In response to legislation to standardize charging ports passed in 2022, Apple said it would comply with regulations. The iPhone 15 in 2023 became the first iPhone to switch to USB-C, and the last few Lightning accessories will make the transition by 2025.