Step 1: Picture the brachiopod shell.
A brachiopod carries two unequal valves, one on top called the dorsal valve and one below called the ventral valve. Each single valve is symmetric left to right, which is a different plan from clams, where the two valves mirror each other.
Step 2: Test each choice against this plan.
Pentameral symmetry is a five-part body plan found in sea stars and crinoids. It has nothing to do with a two-valved brachiopod shell, so this choice is wrong.
The dorsal valve is one of the two shells that make up the animal, and its shape is used all the time to tell species apart. This choice fits.
Thoracic segments describe the middle, jointed part of a trilobite's body. A brachiopod shell carries no segments at all, so this choice is wrong.
Deltidial plates sit near the beak of the ventral valve and cover part of the pedicle opening in many brachiopods. Their shape is a standard feature that taxonomists use, so this choice fits.
Step 3: Collect the fits.
Only the dorsal valve and the deltidial plate describe real brachiopod anatomy.
\[ \boxed{\text{Dorsal valve, Deltidial plate}} \]