The correct answer is C. Create a validation rule that uses the PRIORVALUE function.

The requirement is: once Estimated Hours is set, users can decrease the value but cannot increase it. A validation rule can compare the new value with the old value by using PRIORVALUE().

Example logic:

Estimated_Hours__c > PRIORVALUE(Estimated_Hours__c)

Salesforce documents PRIORVALUE() as the function used to return the previous value of a field, and Salesforce examples use it in validation rules to compare a changed value against the prior value.

A is incomplete because ISCHANGED() only detects that the field changed; it does not determine whether the value increased or decreased.

B is incorrect because a default value does not control later edits.

D is incorrect because PREVGROUPVAL() is used in reports, not validation rules.

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