Double fertilization is a unique reproductive mechanism that occurs in angiosperms, or flowering plants. It involves the fusion of two male gametes with two different female gametes, resulting in the formation of both a zygote and a primary endosperm nucleus. The correct answer is option C, which states that one male gamete fuses with the egg to form the zygote, and the other male gamete fuses with the secondary nucleus to form the primary endosperm nucleus.

The process of double fertilization can be explained in detail as follows:

1. Introduction to Double Fertilization:
- Double fertilization is a key reproductive process that occurs in angiosperms.
- It involves the fusion of two sperm cells with two different female gametes present in the embryo sac.

2. The Structure of the Embryo Sac:
- The embryo sac, also known as the female gametophyte, is a structure found within the ovule of the flower.
- It consists of several cells, including the egg cell, two synergids, three antipodal cells, and a central cell containing two polar nuclei.

3. Pollination and Pollen Tube Formation:
- Pollination occurs when pollen grains are transferred from the anther to the stigma of a flower.
- After pollination, the pollen grain germinates on the stigma and forms a pollen tube.
- The pollen tube grows through the style and reaches the embryo sac.

4. Double Fertilization Process:
- Upon reaching the embryo sac, the pollen tube releases two male gametes.
- One male gamete fuses with the egg cell, resulting in the formation of a diploid zygote.
- This zygote develops into the embryo of the plant.
- The other male gamete fuses with the two polar nuclei present in the central cell of the embryo sac.
- This fusion results in the formation of a triploid primary endosperm nucleus.
- The primary endosperm nucleus develops into the endosperm, which provides nourishment to the developing embryo.

5. Significance of Double Fertilization:
- Double fertilization ensures that the endosperm is formed only when fertilization occurs.
- It allows for efficient allocation of resources by producing the endosperm only when needed.
- The endosperm provides nutrients to the developing embryo, promoting its growth and development.

In conclusion, double fertilization in angiosperms involves the fusion of one male gamete with the egg to form the zygote, and the fusion of the other male gamete with the polar nuclei to form the primary endosperm nucleus. This unique reproductive mechanism ensures the successful development and nourishment of the embryo in flowering plants.