Key Concept: Molecular markers are DNA-level signposts used to track genes for desirable traits in breeding programs.
Why it's correct: SNPs (Single Nucleotide Polymorphisms) are the most abundant, easily genotyped DNA markers and are the backbone of Marker-Assisted Selection (MAS) for traits like milk yield or disease resistance.
Why others fail: Hemoglobin, vitamins, and enzymes are biochemical/physiological molecules, not DNA-based markers — they can't be used the way SNPs are for genomic selection.