Which of the following is monomorphic?
All listed sequence features represent highly variable regions of the genome. Short tandem repeats, insertion-deletion polymorphisms and simple sequence repeats show length or sequence variation among individuals due to replication slippage or transposable element activity. They are extensively used as polymorphic markers because mutation rates are high and multiple allelic forms exist in populations. Monomorphic loci, by definition, show a single invariant allele across all genotypes and lack informativeness for diversity analysis, which does not apply to these hypervariable marker classes.
Ref: NCERT Biology Class XII Principles on Klenow fill-in labeling, Lehninger Chapter 9 DNA cloning techniques, and Molecular Cloning by Sambrook Chapter 10 documenting end-labeling of cohesive termini.