Question

these gravestones have slowly broken down and turned a blackish color due to the action of:
acid rain
abrasion
frost wedging
animal action

Explanation:

Acid rain contains acidic compounds that react with stone (like limestone/marble in gravestones) causing chemical weathering, which breaks down the stone and can leave dark, discolored deposits. Abrasion is physical wear from particles, frost wedging is breakage from frozen water, and animal action does not cause this specific breakdown and blackening.

Answer:

acid rain