Select the correct passive form of the given sentence.

You can see vast valleys sloping into the distance.

Vast valleys can be seen sloping into the distance.

The correct passive form of the given sentence is Vast valleys can be seen sloping into the distance.

The other options are incorrect because:

• Vast valleys had been seen sloping into the distance uses the past perfect tense, while the original sentence is in the present tense.
• Vast valleys are seen sloping into the distance uses the present tense, while the original sentence suggests that the action of seeing the valleys is possible, not certain.
• Vast valleys will be seen sloping into the distance uses the future tense, while the original sentence does not have a future meaning.
• Vast valleys could be seen sloping into the distance is grammatically incorrect. The modal verb "could" is used to express possibility, but it is not used correctly in the passive voice.