Which one of the following guides the entry of the pollen tube into the embryo sac?

The correct option is Micropyle.

In flowering plants (angiosperms), the process of fertilization requires the pollen tube to grow through the style and enter the ovule to reach the embryo sac.
The ovule is surrounded by protective layers called integuments, which leave a small opening or passage at one end. This small opening is known as the micropyle.

The pollen tube enters the ovule specifically through this micropylar opening (a process called porogamy). Therefore, the micropyle serves as the entry point that guides or allows the passage of the pollen tube into the embryo sac.