The correct sentence that uses proper subject-verb agreement is:
The cat carefully steps across the fence.
The other sentences have subject-verb agreement issues:
- "My brother always want to cook dinner." should be "My brother always wants to cook dinner."
- "The clouds swiftly moves across the sky." should be "The clouds swiftly move across the sky."
- "My teachers often gives homework on the weekend." should be "My teachers often give homework on the weekend."