In the folded Appalachians, why can one rock layer stand as a ridge while the layer beside it becomes a valley?

Answer: Some rock layers erode more slowly than the layers next to them

All folded rock layers erode at the same steady rate — No. All the layers do share the same folding history, but they do not wear down at the same rate. If they did, the landscape would not show such sharp parallel ridges and valleys. Differential erosion turns a folded stack into alternating highs and lows, which is why a topography map can hint at the rock types below.

Some rock layers erode more slowly than the layers next to them ✓ — Right. Folding gives the rock stack its striped pattern, but erosion decides which stripes stand high. Resistant sandstone and conglomerate often remain as ridges, while shale, slate, limestone, and carbonate rocks wear down into valleys. The ongoing removal of weaker rock is what leaves the sharp parallel ridges and valleys.

Younger rock layers rise higher than the layers next to them — No. Age does differ from layer to layer, and folding does lift the whole stack. But a layer does not rise simply because it is younger; in the folded Appalachians the present relief mostly reflects which rocks resist erosion. A younger soft layer can be cut down while an older hard layer remains high.