Which process is involved in the formation of seeds without fertilization in flowering plants?

Statement-$X$: Apomixis is seen in a few flowering plants such as some species of $Asteraceae$ and grasses.
Statement-$Y$: Apomixis is a form of asexual reproduction that mimics sexual reproduction.
Statement-$Z$: In some species of apomictic seeds,the diploid egg cell is formed without reduction division.

In citrus,adventive embryony is caused by which of the following?

The formation of an embryo without the fusion of gametes is termed as:

In which of the following species is nucellar polyembryony observed?

What is apomixis and what is its importance?