Final answer:
The researcher is using a longitudinal design to study language development in infants over a span of five years.
Step-by-step explanation:
The research design described in the question is an example of longitudinal design. Longitudinal research involves following a group of research participants for an extended period of time and collecting data from them at regular intervals. In this case, the researcher is studying language development in a specific group of infants starting from 6 months old and continuing at 6-month intervals for the next five years. This design allows the researcher to observe how language skills develop and change over time in the same group of individuals.