The problem asks to identify which lines in the cube are intersecting, based on the given options. Let's analyze the cube and the lines mentioned:

Cube vertices:

• O (top-left-back)
• P (top-right-back)
• Q (top-right-front)
• R (top-left-front)
• S (bottom-left-back)
• T (bottom-right-back)
• U (bottom-right-front)
• V (bottom-left-front)

Line Segments:

1. QU: Connects the top vertex $Q$ to the bottom vertex $U$ (vertical line).
2. ST: Connects the bottom vertices $S$ and $T$ (horizontal line at the bottom).
3. RV: Connects the top vertex $R$ to the bottom vertex $V$.
4. PT: Connects the top vertex $P$ to the bottom vertex $T$.
5. OP: Connects the top vertices $O$ and $P$ (horizontal line at the top).
6. OS: Connects the top vertex $O$ to the bottom vertex $S$.

Analyzing the options:

• QU and ST: These lines do not intersect. $QU$ is vertical, and $ST$ is horizontal at the bottom of the cube, but they are not in the same plane.
• ST and RV: These lines do not intersect. $ST$ is horizontal at the bottom, and $RV$ is vertical in a different plane.
• PT and OP: These lines are on different planes. $PT$ is diagonal inside the cube, while $OP$ is along the top face.
• OS and PT: These two lines intersect at point O.

Thus, the correct answer is OS and PT, as these lines meet at point $O$.

Would you like more details or have any other questions?

Related questions:

1. What are the conditions for two lines to be classified as intersecting in 3D geometry?
2. What is the difference between skew lines and intersecting lines?
3. Can two parallel lines ever intersect in Euclidean geometry?
4. How can you determine if two lines are parallel just by looking at a 3D diagram?
5. What are coplanar lines, and how do they relate to this problem?

Tip: In 3D geometry, lines that are neither parallel nor intersecting are called skew lines, as they exist on different planes and never meet.