Who coined the term linkage?

The correct answer is Option (3) → Thomas Hunt Morgan.

Thomas Hunt Morgan made many dihybrid crosses in the fruit fly Drosophila. He found that when two genes lie on the same chromosome, the parental combinations of traits appear far more often than the new combinations. He called this physical association of genes on a chromosome linkage. He used the term recombination for the formation of non-parental gene combinations.

NCERT Extract:
Linkage and Recombination – “… Morgan and his group knew that the genes were located on the X chromosome (Section 4.4) and saw quickly that when the two genes in a dihybrid cross were situated on the same chromosome, the proportion of parental gene combinations were much higher than the non-parental type. Morgan attributed this due to the physical association or linkage of the two genes and coined the term linkage to describe this physical association of genes on a chromosome and the term recombination to describe the generation of non-parental gene combinations (Figure 4.11).”