Phenomenon leading to variation in DNA:

Mutation:
- Mutation is a spontaneous change in the DNA sequence that can lead to genetic variation. It can occur due to errors in DNA replication, exposure to mutagens, or environmental factors.
- Mutations can be beneficial, neutral, or harmful, and they can result in different traits or characteristics in an organism.

Recombination:
- Recombination is the process where DNA segments are exchanged between chromatids during meiosis. This leads to the creation of new combinations of alleles on a chromosome.
- Recombination increases genetic diversity by shuffling genetic material from two parents, leading to offspring with unique combinations of genes.

Linkage:
- Linkage refers to the tendency of genes that are located close to each other on a chromosome to be inherited together. Linked genes do not assort independently during meiosis.
- Linkage can lead to variation by keeping certain combinations of alleles together, limiting the possible genetic combinations that can be produced.

In conclusion, mutation and recombination are the main phenomena that lead to variation in DNA. Mutations introduce new genetic changes, while recombination generates new combinations of alleles. Linkage, on the other hand, can influence the inheritance of certain gene combinations but may not contribute as directly to genetic variation.

Option (b)..