What is defined as a frame where properties like opacity or position can be modified?

The correct choice is keyframe. A keyframe is a specific point in time in a video clip where a change occurs in properties such as opacity, position, scale, and other effects. When you place a keyframe in your timeline, you are essentially specifying a state of an element at that exact moment. Premiere Pro uses keyframes to create animations or transitions by interpolating values between keyframes, allowing for smooth changes over time.

For instance, if you want to animate the position of a video clip, you would add a keyframe at the point where you want the position to start changing and another keyframe where you want it to end. Premiere Pro will then automatically calculate the intermediate positions, resulting in a fluid motion.

The other options have different meanings. A clip refers to a video or audio piece in the timeline. A marker is a note or reference point within the timeline but does not affect properties directly. A segment generally refers to a portion of a timeline or a defined part of a project but does not specifically represent the modification of properties.