Both plagiarism and purchasing an assignment and submitting it as your own work are considered serious breaches of academic integrity. However, the most clear-cut answer from the options provided is:
Plagiarism (passing off someone else’s words or ideas as your own)
This is traditionally viewed as a fundamental violation of academic integrity because it directly involves misrepresenting someone else's work as your own.
Purchasing an assignment and submitting it as your own also qualifies as a serious breach, but if you have to choose one based on the typical emphasis in academic integrity materials, plagiarism is often highlighted as particularly egregious.