The polar nuclei and male gamete fuse to form

The correct option is PEN.

To understand why this is the correct answer, let's look at the process of double fertilization in angiosperms (flowering plants):
1. Double Fertilization: Flowering plants undergo a unique process where two fertilization events occur within the embryo sac.
2. First Fertilization (Syngamy): One haploid male gamete fuses with the haploid egg cell to form a diploid zygote, which eventually develops into the embryo.
3. Second Fertilization (Triple Fusion): The second haploid male gamete moves towards the center of the embryo sac and fuses with the diploid secondary nucleus (which is formed by the fusion of two haploid polar nuclei).
4. Formation of PEN: Because this second fusion involves three haploid nuclei (one male gamete and two polar nuclei), the process is called triple fusion. The fusion of these nuclei results in the formation of a triploid (3n) nucleus called the Primary Endosperm Nucleus, commonly abbreviated as PEN.

Therefore, the direct product of the fusion between the polar nuclei (secondary nucleus) and the male gamete is the Primary Endosperm Nucleus (PEN).