Incomplete Octet:

An incomplete octet refers to a situation where an atom does not have a complete octet of electrons in its valence shell. This typically occurs with elements from Group 13 to Group 17 of the periodic table.

Explanation of options:

a) NH3 (Ammonia):
In NH3, the central atom is nitrogen (N). Nitrogen has 5 valence electrons and forms three covalent bonds with three hydrogen atoms. This allows nitrogen to achieve a complete octet by sharing three electrons. Therefore, NH3 does not have an incomplete octet.

b) BCl3 (Boron Trichloride):
In BCl3, the central atom is boron (B). Boron has 3 valence electrons and forms three covalent bonds with three chlorine atoms. However, boron only has 6 electrons around it, which is less than the octet. Therefore, BCl3 has an incomplete octet.

c) CCl4 (Carbon Tetrachloride):
In CCl4, the central atom is carbon (C). Carbon has 4 valence electrons and forms four covalent bonds with four chlorine atoms. This allows carbon to achieve a complete octet by sharing its electrons. Therefore, CCl4 does not have an incomplete octet.

d) PCI3 (Phosphorus Trichloride):
In PCI3, the central atom is phosphorus (P). Phosphorus has 5 valence electrons and forms three covalent bonds with three chlorine atoms. However, phosphorus only has 6 electrons around it, which is less than the octet. Therefore, PCI3 has an incomplete octet.

Conclusion:

Among the given options, BCl3 and PCI3 have incomplete octets. BCl3 has an incomplete octet because boron only has 6 electrons around it, while PCI3 has an incomplete octet because phosphorus only has 6 electrons around it. The other options, NH3 and CCl4, have complete octets and do not have an incomplete octet.