Q.A dikaryon is formed when

Option (B) is correct: a dikaryon forms when the two haploid cells do not fuse immediately.

• In most fungi, fusion of two haploid cells results straight away in a diploid (2n) cell.
• In ascomycetes and basidiomycetes, however, an intervening stage occurs in which the cytoplasms have fused but the two nuclei remain separate within one cell — this n+n condition is the dikaryon, and the phase is called the dikaryophase. …

When fungi reproduce sexually, two compatible haploid hyphae come together and fuse. In many fungi this fusion of haploid cells immediately produces a diploid (2n) cell. But in ascomycetes and basidiomycetes, the process pauses partway: the cells fuse (cytoplasmic union has taken place) yet their two nuclei stay distinct within the same cell, giving an n+n condition with two nuclei per cell rather than one fused nucleus. This condition is called a dikaryon, and the phase during which it persists is the dikaryophase. Only later do the parental nuclei actually fuse, at which point the cell becomes truly diploid. …