Answer:
Undercoverage
Step-by-step explanation:
using the telephone directory as the source of survey subject could lead to undercoverage because the sole reason for carrying out a survey is to select a sample that represent the whole populace not a sample that represent those that have telephone.
undercoverage will be a constraint because people without a telephone or people with telephone number not on the telephone directory were not observe in the sample frame and could lead to confounding.