The scientists who have given the theory of linkage are:
(a) Morgan and castle
(b) Beadle and Tatum
(c) Watson and Crick
(d) Bateson and Punnett

Hint: Morgan and Castle proposed the 'Chromosomal Theory of Linkage' in 1911.

Complete answer:In 1911, T. H. Morgan and his co-researcher W.E. Castle made an experiment with Drosophila melanogaster. According to this theory, genes are arranged in a linear manner on a chromosome. The genes present on the same chromosome have a tendency to be inherited together. This is known as linkage.

Hence, the genes on the same chromosome show linkage. Such genes are known to be linked genes. They have a tendency to remain in parental combinations. Rarely these allow crossing over to happen. The strengths of linkages can also be determined by the distance between the linked genes. The closer they reside, the stronger the linkage is. Distantly located genes exhibit weaker linkage.

The distance calculation is often done by scientists and students, this sort of calculation is called ‘linkage mapping’. The above mentioned conclusions of the ‘Chromosomal Theory of Linkage’ were obtained of various crossing experiments done by the scientists. When crossing over rates are below 50%, linkage is observed. If crossing over rates exceed the value of 50%, no linkage exists.

Beadle and Tatum gave the ‘One Gene One Enzyme’ hypothesis; Watson and Crick gave the DNA model. Bateson and Punnet gave the ‘Coupling and Repulsion’ hypothesis.
So, the correct option is A. Morgan and Castle.

Note:T.H. Morgan has done several experiments on Drosophila and gave various concepts of genetics. But linkage theory was put forward by Morgan and Castle together.