The nature of the polymorphic DNA fragment used for mapping is co-dominant.

Explanation:
- Polymorphic DNA fragments are used in genetic mapping to identify variations in DNA sequences between individuals or populations.
- These variations are caused by differences in the DNA sequence, such as single-nucleotide polymorphisms (SNPs) or insertions/deletions (indels).
- The co-dominant nature of a polymorphic DNA fragment means that both alleles (alternative forms of a gene) are expressed equally in heterozygous individuals.
- In other words, both the paternal and maternal alleles are observed separately in the DNA fragment.
- This allows for the detection of two distinct bands or peaks on a gel or chromatogram, corresponding to the two different alleles.
- Co-dominant markers are widely used in genetic mapping studies because they provide more information than dominant markers.
- Dominant markers, on the other hand, only show the presence or absence of a particular allele, without distinguishing between homozygous and heterozygous individuals.
- Partial dominance is a concept related to the expression of traits in individuals heterozygous for a particular gene, and it is not directly applicable to DNA mapping.
- Recessive markers are also not applicable in this context, as they do not allow for the observation of both alleles in heterozygous individuals.
- Therefore, the correct answer is option 'C': co-dominant.