Step 1: Picture each layout.
Draw the four layouts in your head. Ring is a circle of nodes. Star is a hub with spokes. Mesh is a web of many links. Bus is one straight line with nodes hanging off it.
Step 2: Match the clue word.
The clue is a backbone wire. A backbone is the main line of a body or a network. Only the straight-line layout has one main line that carries everything.
Step 3: Rule out ring.
Ring has no single main line. Each cable joins just two neighbouring nodes. So ring is not the answer.
Step 4: Rule out star and mesh.
Star uses one cable per node going to a central device. Mesh uses many point-to-point cables. In both, the cables are not shared by all nodes. So these two are not the answer.
Step 5: Accept bus.
Bus topology is built on one shared cable that every node connects to, so it matches the clue. The correct choice is option 2.
Step 6: Extra check.
Many students confuse bus with star because both look like lines drawn from a centre. The difference is the cable. In a star every node has its own cable to a hub. In a bus there is only one cable, shared by all nodes, and a break in it can stop the whole network. Keeping this point in mind makes the answer sure.
Final Answer:
Using a shared backbone cable, the topology is Bus, option 2.
\[ \boxed{\text{Option 2: Bus}} \]