Researchers repeatedly present Jenna, a one-month-old infant, with a sound ('puh') while measuring the rate at which she sucks on her pacifier. When her sucking rate slows, they begin to present Jenna with a different sound ('buh'). The researchers are most likely testing Jenna's

Syntactic awareness involves understanding the rules that govern sentence structure and word order. This skill typically develops later in infancy and childhood, as children begin to construct and comprehend more complex sentences. Since Jenna is only one month old and the study focuses on sound differentiation rather than sentence structure, this option does not apply.

The Moro reflex is an involuntary response seen in infants, characterized by spreading out the arms and then retracting them when startled. This reflex is not related to sound discrimination and is more about physical reactions to stimuli, making it irrelevant to the context of this study on auditory processing.

Visual acuity refers to the clarity or sharpness of vision and is unrelated to auditory processing. The study centered around sound stimuli and sucking behavior, so visual acuity does not pertain to the researchers' objective of testing sound discrimination.

Receptive vocabulary refers to the words an individual can understand when heard. While related to language development, it encompasses a broader range of words rather than specific sound differences. The focus of the study is on Jenna's ability to distinguish between specific phonemes, not her understanding of words.