Final answer:
XYZ Accounting's 15-year negligence in estimating Cliff's income would cover the cost of the lawsuit.
Step-by-step explanation:
The cost of the lawsuit for XYZ Accounting would be covered by option C: XYZ Accounting's 15-year negligence in estimating Cliff's income.
In this scenario, Cliff owes $15,000 in back taxes due to the severe underestimation of his Adjusted Gross Income by XYZ Accounting for the past year. Cliff decides to sue XYZ Accounting for this mistake. XYZ Accounting's negligence in estimating Cliff's income for 15 years would be the primary cause of the error, making them responsible for the consequences.
Therefore, option C, XYZ Accounting's 15-year negligence in estimating Cliff's income, is the most suitable option to cover the cost of the lawsuit.