From an internal control standpoint, the asset most susceptible to improper diversion and use is Cash.
What is cash?
Cash is the most liquid asset because it can easily be used in exchange unlike other assets.
Cash is classified under the Current Assets section of the balance sheet but is always presented first before debtors, inventory, and short-term investments.
Thus, from an internal control standpoint, the asset most susceptible to improper diversion and use is Cash.