During meiosis I , in which stage synapsis takes place?

Prophase I is subdivided into five phases, viz. Leptotene, Zygotene, Pachytene, Diplotene and Diakinesis. 

Leptotene: During this stage, the chromosomes become gradually visible under a light microscope. Compaction of chromosomes continues throughout this phase. 

Zygotene: Chromosomes start pairing together. This process is called synapsis. The paired chromosomes are called homologous chromosomes. The formation of synapsis is accompanied by the formation of the synaptonemal complex. 
Pachytene: Recombination nodules appear. These nodules are the sites at which crossing over takes place between non-sister chromatids of the homologous chromosomes. 
 
Diplotene: Synapotnemal complex is dissolved at this stage. The recombined homologous chromosomes of the bivalent separate from each other; except at the site of crossing over. The X-shaped structures; thus formed; are called chiasmata. 

Diakinesis: Chiasmata is terminated at this stage.