Formation of dikaryon is characteristic of

The correct option is Both (b) and (c).

To understand why this is correct, let us break down the process of sexual reproduction in fungi:

1. Understanding Dikaryon:
In many fungi, sexual reproduction involves the fusion of two compatible haploid hyphae. Typically, the sexual cycle consists of three distinct phases:
- Plasmogamy: The fusion of protoplasm between two motile or non-motile gametes.
- Karyogamy: The fusion of the two haploid nuclei to form a diploid nucleus.
- Meiosis: Cell division that reduces the chromosome number by half, producing haploid spores.

2. Immediate Fusion vs. Delayed Fusion:
In primitive fungi, such as Phycomycetes, plasmogamy is immediately followed by karyogamy. The two nuclei fuse right away to form a diploid zygote, which is represented as:

$2n$

However, in more advanced fungi, specifically Ascomycetes (sac fungi) and Basidiomycetes (club fungi), karyogamy is delayed. After the fusion of cytoplasm (plasmogamy), the two nuclei do not fuse immediately. Instead, they remain paired within the same cell, resulting in a condition where each cell contains two distinct nuclei. This specific stage is represented as:

$n+n$

This condition is called a dikaryon (or dikaryotic phase), and the phase is referred to as the dikaryophase of the fungus. Later, when these fungi form their respective fruiting bodies, karyogamy occurs, and the nuclei fuse to form diploid cells that undergo meiosis to produce spores.

Conclusion:
Since this unique dikaryotic phase is a key characteristic of both Ascomycetes and Basidiomycetes, the correct answer is Both (b) and (c).