The correct option is
C Primary endosperm nucleus
PEN
PEN stands for Primary Endosperm Nucleus.
One of the two male gametes released from the pollen tube fertilises the diploid central cell of the embryo sac to produce a triploid primary endosperm cell (PEC) during double fertilisation.
This results in the formation of the primary endosperm nucleus (PEN) inside the PEC. The PEN formed due to fusion of three nuclei (male gamete nucleus and secondary nucleus formed out of two polar nuclei) is considered triploid. Hence, the process is referred to as triple fusion. The PEC with the PEN divides further and develops into the endosperm.
Final answer
(b) Primary endosperm nucleus