In which situation will river water be less useful for transport?

Correct answer: Very low depth and many obstacles

The direct answer is A: very low depth and many obstacles. River transport needs enough water depth for boats, a reasonably safe channel and few obstructions. The reasoning is simple: a boat must float without touching the bottom; obstacles can damage it or block its route; therefore shallow, obstructed water is difficult and unsafe for navigation. Option A is correct because both conditions directly prevent or restrict boat movement. Option B is wrong because adequate depth and gentle flow make travel easier and safer. Option C is not the best answer because a port is a helpful facility, not a condition that makes water less useful. Option D is also helpful, since continuous water supports regular transport. Exam cue: navigation requires depth, a usable channel and manageable flow.