To determine the domain of the function based on the condition given (a customer's account will lock if they owe more than $100), we need to consider what values of \( x \) (the balance of the customer's account) will cause the function to be defined.
A customer owes more than $100 if their balance is less than -$100. This means that the account will not lock for balances greater than or equal to -$100, including all balances that are $0 or more (positive balances) as well as any negative balance that is less than -$100.
Given this, we can conclude that the balance could range from -$100 to positive infinity. Thus, the domain of this function would be represented as:
\[ [-100, \infty) \]
Therefore, the correct choice is: [−100,∞].