CSIR NET June 2016 Q58: Sedimentology
Sea-level fall during the Last Glacial Maximum (LGM, ~18-20 ka) exposed continental shelves and steepened river gradients, triggering fluvial incision.
During the LGM (~18 ka), global sea level fell by ~120 m, exposing the continental shelf. Rivers extended their courses across the exposed shelf and, responding to the new, steeper base-level gradient, incised into their earlier floodplains/valley fills. This incision is recorded as buried channels and terraces beneath present shelves and lower reaches. Option 3 is correct.
1 and 2) Incorrect — sea level rise (post-glacial) causes aggradation/backfilling, not incision. 3) Correct — the LGM sea-level lowstand is the well-documented cause of shelf-river incision. 4) Incorrect — tectonic uplift is a separate, non-eustatic cause not indicated here.