In the diploid nucleus, each chromosome is represented twice: by a homologous pair and there are same genes on homologous chromosomes.
In Drosophila, there are four pair of chromosomes: 2 pairs of large autosomes, (one slightly smaller than the other pair), 1 pair of very small autosomes, and a pair of sex chromosomes. Genes present on one chromosome form a linkage group. Homologous chromosomes carry same set of genes (the alleles may differ), hence same linkage group.
So, the correct answer is 'Four'.