How does the egg of a flower become fertilized? 1 answer, not multiple choice.
as soon as the pollen reaches the stigma
after the pollen grain descends down the style
when the pollen tube grows into the ovule
when the seed germinates

as soon as the pollen reaches the stigma

Explanation:

When pollen lands on stigma, it germinates and gives rise to a pollen tube that passes through the style and reaches the ovary of a pistil

when the pollen tube grows into the ovule

Explanation:

Fertilization in plants occurs after pollination.  Pollination does not always guarantee fertilization. Whether fertilization will occur or not totally depends upon the later stages which occur after pollination.

The following stages give a better idea of how egg of flower is fertilized:

1) Pollination: A soon as a pollen reaches the stigma pollination is said to occur.

2) Germination: When pollen comes in contact with stigma, it absorbs some chemicals like inositol, Ca²⁺, boran, brassinosteroids  and water from the stigma as a result of which pollen starts to increase in size to form pollen tube. The pollen tube begins to grow which is also known as germination.

3) Penetration of ovule & 4) Fertilization: The growing pollen tube travels through style to reach ovary which has ovule inside it. The pollen contains 2 haploid nuclei while ovule contains 3 haploid nuclei. One of the nuclei in ovule is in the egg cell so it is haploid while 2 nuclei are inside central cell. When the pollen tube keeps on increasing in size, it ultimately penetrates the ovule and one of the nuclei of pollen tube fuses with egg cell and the process is known as fertilization.The other nuclei of pollen tube fuses with central cell which ultimately has 3 nuclei and the process is known as triple fusion. These two processes i.e. fertilization & triple fusion are collectively known as double fertilization.