A valid Lewis structure of which compound cannot be drawn without violating the octet rule?

The correct answer is c) PO₄^3-.

Explanation: The octet rule states that atoms tend to form compounds in ways that give them eight electrons in their valence shell, achieving a stable electron configuration similar to that of noble gases. In considering the compounds listed:

• CF₄: Carbon can form four bonds with fluorine, fulfilling the octet rule with 8 electrons around carbon.
• SeF₄: Selenium can expand its octet and accommodate more than 8 electrons, hence it does not violate the octet rule.
• PO₄^3-: Phosphorus can have more than 8 electrons, but in certain states and resonance forms, its bonding creates situations where it is unable to satisfy the octet for all oxygen atoms, leading to violations of the octet rule.
• NF₃: Nitrogen satisfies the octet by forming three bonds with fluorine.
• SiF₄: Silicon forms four bonds and satisfies the octet rule.

Therefore, among the given options, PO₄^3- is the one where drawing a valid Lewis structure leads to violations of the octet rule due to the requirement for phosphorus and its bonding capabilities.