Option-wise explanation

(1) Polymorphism in intronic regions of a gene cannot be used for trait mapping. – Incorrect

Any heritable DNA polymorphism that can be genotyped and segregates in a population can serve as a marker, regardless of whether it is in an exon, intron, UTR, or intergenic region.
Intronic SNPs or length variants are widely used as markers tightly linked to (or within) genes controlling traits. Saying they “cannot be used for trait mapping” is therefore wrong.

(2) Codominant molecular markers can be used to detect heterozygosity. – Correct

Codominant markers (e.g., SSRs, many SNPs, RFLPs) distinguish both homozygotes and the heterozygote genotype (AA, Aa, aa).
Because heterozygotes show both allelic states, these markers are ideal for detecting heterozygosity in breeding and mapping populations.

(3) STMS and SSRPs are based on polymorphisms in repetitive DNA sequences. – Correct

Sequence Tagged Microsatellite Sites (STMS) and Simple Sequence Repeat Polymorphisms (SSRPs) both target microsatellites, which are short tandemly repeated motifs (e.g., (CA)n(CA)n).
Polymorphism arises from differences in repeat number, so they are explicitly based on repetitive DNA sequences.

(4) RFLPs and SSRs are multi-allelic markers. – Correct

• RFLPs: Different restriction sites or fragment sizes at the same locus can generate more than two alleles in a population (e.g., several distinct fragment patterns).

• SSRs: Variation in repeat number typically produces multiple alleles (several different fragment sizes) at one locus.
  Hence both are commonly described as multi-allelic markers.

Because (2), (3), and (4) are correct, the only INCORRECT statement is option (1).