a biologist wants to classify protein secretion into groups based on the principal function of the protein. which cell works the biologist select to study and why?

The biologist should have a look at the structure of the protein. Since all functions are largely dependent on the structure of the protein (another way to look at it would be that proteins are specialized to carry out their functions because of how they are structured), this is determined by the amino acid sequence of the protein. And the sequence of amino acids is given by the DNA sequence, so that is what the biologist should look at.

Hope that answers the question, have a great day!