C

Reasoning: To compare the evolving fat composition of breast milk from the same mothers of pre-term and term infants, samples must be collected repeatedly over the course of lactation; such repeated measurements on the same participants constitute a longitudinal (prospective observational) study design, which is exactly how similar research has been conducted [Gates, 2021, PMID 34293087; Bauer, 2011, PMID 20801561].