b. stratigraphically alternating sequences of sandstones, conglomerates, and shales.
Explanation:
It is unlikely that one would find igneous rocks such as basalt and granite in fluvial environments. Coal is also not usually found in fluvial environments. Limestone prefers shallow and calm waters so would not be moved by water normally. This therefore invalidates options a, c, d, e and f.
For the hypothesis to be true, one would need to find conglomerates as these contain large clasts and are present in fluvial environments. Alternating sequences involving conglomerates and sandstones would indeed show that that some floods were able to carry large clasts.
