Mendel found certain traits not to assort independently. It is due to
A. Dominance
B. Linkage
C. Crossing over
D. Amitosis

Hint: When two pairs of traits are combined in hybrid, segregation of one pair of traits is independent of the other pairs of traits and it is known as the law of independent assortment. This law is based on dihybrid cross.

Complete answer:
Linkage is the physical association of two genes present on the similar chromosomes. According to the Mendel’s law of independent assortment, the gene controlling different characters gets assorted independent to each other (if the genes are present on two different chromosomes) but if these genes are present on the same chromosomes they may or may not show independent assortment. If the crossing over takes place between two genes then the genes get segregated and they will assort independent to each other but if there is no crossing over between two genes there is no segregation, hence only parental combination will be found in gametes. Linkage is performed by Morgan.
Dominance is the phenomenon in which two allelic forms of a gene and one is expressed in F1 generation and this gene is dominant over another one. The other gene is called recessive gene.
Crossing over is the exchange of chromosome (genetic material) segments during sexual reproduction between two homologous chromosomes.
Amitosis is direct cell division or binary fission. It is cell proliferation that does not occur by mitosis. This method is essential for cell division in eukaryotes.

So the correct answer is option B.

Note: Morgan performed several dihybrid crosses in Drosophila to study genes that were X-linked and these crosses are similar to dihybrid crosses carried out by Mendel in peas. Crossing over is also known as chromosomal crossover. The law of independent assortment is not true for the linked genes.