Final answer:
A tree doctor's use of twenty or more tree names is referred to as his focal vocabulary, as it represents the specialized terms used within his profession for effective communication.
Step-by-step explanation:
When discussing a tree doctor who uses twenty or more names of trees, we refer to his B. focal vocabulary. A focal vocabulary is a set of specialized terms and expressions used within a particular profession or hobby that allows for precise and efficient communication. For example, a tree doctor, or arborist, will have a unique set of terms for various species of trees and processes related to their care, just as different cultures have specific vocabularies for classifying and discussing elements vital to their daily lives, like plants or animals. This is distinct from style shifts, protolanguage, call systems, and productivity, which describe other aspects of language use and development.