Option‑wise explanation

(1) AFLP

AFLP (Amplified Fragment Length Polymorphism) is a dominant, gel‑based fingerprinting technique.
It can detect polymorphisms without prior sequence information, but it samples only a modest fraction of the genome and yields a limited number of anonymous bands, which is often insufficient to resolve very closely related accessions with minimal variation.

(2) GBS – correct

GBS is a reduced‑representation, sequencing‑based approach that uses restriction enzymes plus next‑generation sequencing to sample thousands to millions of loci across the genome.
Because it directly discovers and scores SNPs at very high density, it is particularly suitable for uncovering polymorphism between closely related genotypes where older marker types may show little or no variation.

(3) SSR

SSRs (microsatellites) are co‑dominant and informative but are locus‑specific and relatively few in number in a typical panel.
Finding polymorphic SSRs between very similar accessions often requires testing many markers, and even then genome coverage is sparse compared with GBS.

(4) RAPD

RAPD markers are dominant, low‑reproducibility, random‑amplification markers.
They are generally considered obsolete for fine discrimination because banding patterns are sensitive to reaction conditions and the number of scorable polymorphisms is small, making them poorly suited for very closely related accessions.

Therefore, for discovering polymorphism between two closely related accessions, GBS (option 2) is the most appropriate and powerful approach.