Total no. of nuclei involved in double fertilization in a normal diploid plant is (JRF Plant Science 2019)

✅ Explanation:
Double fertilization in angiosperms involves the fusion of one sperm nucleus with the egg cell (to form the zygote) and another sperm nucleus with the two polar nuclei (to form the endosperm). So, a total of 5 nuclei are involved: 2 sperm nuclei, 1 egg nucleus, and 2 polar nuclei.

🔑Key Points:
Double fertilization is a distinct mechanism of fertilization observed only in angiosperms or flowering plants. In the process of double fertilization, one male gamete (sperm cell) from the pollen joins with the egg cell (female gamete) to form the zygote, which develops into the embryo. Meanwhile, the second male gamete fertilizes the secondary nucleus, known as the central cell nucleus, to form the endosperm nucleus. This endosperm nucleus develops into the endosperm, which serves as a nutrient store for the developing plant embryo. Therefore, the male gamete and the secondary nucleus give rise to the endosperm in double fertilization. This is a unique and characteristic feature of angiosperms, or flowering plants. Here's the detailed explanation of the process:  
-Pollen Transfer: The process begins with the transfer of pollen from the male part of the flower, known as the anther, to the female part of the flower, known as the stigma. This process is called pollination, and it can occur through various agents like wind, water, or pollinators such as bees and butterflies.  
-Pollen Tube Formation: Once the pollen grain lands on the stigma, it begins to germinate by growing a pollen tube down through the style (the long extended part of the female organ) towards the ovule inside the ovary.  
-Sperm Transfer: Each pollen grain contains two sperm cells. As the pollen tube extends, it carries these two sperm cells down into the ovary.  
-Double Fertilization: When the pollen tube reaches one of the ovules inside the ovary, it releases the two sperm cells into the ovule. One sperm cell then fertilizes the egg cell to produce a zygote, which will develop into the future plant embryo. The second sperm cell fuses with the two polar nuclei or secondary nuclei present in the central cell, forming a triploid cell (because it has three sets of chromosomes). This cell will develop into the endosperm, a tissue rich in nutrients that will feed the developing embryo during seed formation.  
-Seed and Fruit Formation: After double fertilization, the zygote begins to grow into an embryo within the fertilized ovule. Meanwhile, the embryo sac develops into the seed, and the surrounding ovary tissue develops into the fruit that encloses the seed.