Question

which surface of the leaf has more prominent viens

Answer 1

upper surface of the leaf has prominent veins because there is more requirement of water on the upper surface of the leaf as the exposure of Sunlight is more on the upper surface and chlorophyll is also more presents on the upper surface (bcz of which the leaves are dark green on upper side then lower side).
So for high rate of photosynthesis there is more supply of water on the upper surface .