Final answer:
Calcium carbonate is the most abundant compound that accumulates in river deltas.
Step-by-step explanation:
The most abundant compound that accumulates in river deltas is calcium carbonate (b) .
When rivers flow into the ocean, they carry sediments and dissolved compounds with them. Over time, these sediments accumulate in river deltas, which are areas of land where the river meets the ocean. One of the main compounds found in these sedimentary deposits is calcium carbonate. Calcium carbonate is formed through chemical reactions between dissolved calcium ions and bicarbonate ions in the water.