If fossils of two closely related species A and B are found in the sedimentary rocks of early Triassic stratum, the fossil of their most recent common ancestor is most likely to be found in

The sequence of periods from oldest to most recent is: 

Palaeozoic Era – Cambrian, Ordovician, Silurian, Devonian, Carboniferous, Permian 

Mesozoic Era – Triassic, Jurassic, Cretaceous

Coenozoic Era – Tertiary, Quaternary

The Permian period is just before the Triassic period. The most common recent ancestor of (A) and (B)  is most likely to have lived during the Permian period.