The chromosome number is reduced in which phase of meiosis?

$A$: Meiosis occurs only in diploid $(2n)$ cells.
$R$: Meiosis-$I$ is known as reductional division.

During which phase do homologous chromosomes align at the equatorial plate of the cell?

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:

Assertion : During zygotene,chromosomes show bivalent stage.
Reason : Bivalent is half the number of chromosomes.

Assertion : Diplotene is characterized by the presence of chiasmata.
Reason : Diplotene can last for months and years in oocytes of some vertebrates.