At which stage does the number of homologous chromosomes become halved?

In meiosis,the number of chromosomes is .....

Which of the following statements are correct regarding meiosis?

Given below are two statements:
Statement $I$: Chromosomes become gradually visible under light microscope during leptotene stage.
Statement $II$: The beginning of diplotene stage is recognized by dissolution of synaptonemal complex.
In the light of the above statements,choose the correct answer from the options given below:

The non-sister chromatids twist around and exchange segments with each other during

Haploid $(n)$ cell formation from diploid $(2n)$ cell is known as.......