Single nucleotide polymorphisms (snps) are useful in epidemiology because:
a. it is a laboratory technique that permits manipulation of dna to determine gene function
b. their presence is evidence of a defect in dna replication, and therefore proof that a given disease is present
c. they may be located within or in close proximity to a gene, and may therefore serve in identifying carriers of a characteristic (including a genetic defect)
d. it is an approach to determine the location of a given gene, therefore speeding identification of the mechanism of the disease and confirming carriers of the defect

b. Their presence is evidence of a defect in DNA replication, and therefore proof that a given disease is present

Explanation:

Single nucleotide polymorphisms (SNPs) result from point mutations and correspond to the position where there is alternation of nucleotides A, C, G, T at a minimum allele frequency of 1% in a given population. SNPs can be present in both coding and non-coding regions of the genome. As a nucleotide alternation is the result of a defect in DNA replication and these defects cause syndromes and diseases; we can say that single nucleotide polymorphisms are evidence of a defect in DNA replication and therefore proof that a particular disease is present.

you didn't give any options but heres what i can tell you: an inference is likely to change based on additional observations. the latter is something that you have seen or sensed using your sensory organs while the former is like a conclusion out from what you have observed.