Final answer:
d. Polar neutral
The classification of modified tryptophan residues after photochemical modification is uncertain without further experimental data or context. Original tryptophan is nonpolar and hydrophobic, but modifications could alter its properties.
Step-by-step explanation:
The question pertains to the classification of amino acids, particularly the classification that applies to tryptophan (Trp) residues after photochemical modification by CCl₃CO₂H. Amino acids can be broadly classified into acidic, basic, hydrophobic, or polar neutral, depending on the nature of their side chains. Tryptophan is originally a nonpolar, hydrophobic amino acid due to its indole ring in the side chain. However, after photochemical modification with a strong oxidant like CCl₃CO₂H (trichloroacetic acid), which is likely to modify the indole ring, we need to consider how the modification changes the chemical properties of the side chain. It is possible that the Trp side chain could add chloro groups or other modifications which might increase its polarity but without further specifics, it's difficult to definitively classify the modified residues without experimental data. Thus, it's not possible to precisely classify the Trp residues after such modification without additional context. In the absence of more information, we must refuse to answer.