Answer:
A
Explanation:
The correct answer would be that Watson and Crick's structure of DNA was not done until 1953.
The fact that chromosomes are condensed together in the nucleus, are not distinguishable during interphase, and a method had not yet been devised to halt mitosis at metaphase could have plausibly delayed the successful counting of human chromosomes.
On the other hand, the discovery of the structure of the DNA could not have delayed the study of chromosomes because the DNA structure has no bearing on the karyotyping of chromosomes.
The chromosomes of organisms are best studied using a relevant microscope during metaphase when they would have fully decondensed and align at the equator of the cell or the metaphase plate. At the interphase phase of the cell cycle, chromosomes are full condensed under the view of the microscope.
The correct option is, therefore, A.
