A validation rule is the appropriate tool to enforce data integrity conditions before a record is saved. The requirement is to compare the field's new value with its previous value during an update. The PRIORVALUE function is specifically designed for this purpose within a validation rule. The formula EstimatedHoursc > PRIORVALUE(EstimatedHoursc) will evaluate to true only when a user attempts to enter a value greater than the one previously saved, thereby triggering the validation error message and preventing the record from being saved.

A. Create a formula default value for the custom field.

A default value only populates a field when a new record is created. It does not control or validate changes made to the field on existing records.

B. Create a formula field that uses the PREVGROUPVAL function.

The PREVGROUPVAL function is used exclusively in reports and dashboards to compare summary values between different groupings. It cannot be used in a validation rule for record-level data validation.

C. Create a validation rule that uses the ISCHANGED function.

The ISCHANGED function only determines if a field's value has been modified. It cannot compare the new value to the old value to determine if it was an increase or decrease.

1. Salesforce Help Documentation, "PRIORVALUE": "Use this function in validation rules... to compare the new value of a field to the previous value." The documentation provides the exact syntax and use case for comparing old and new field values.

2. Salesforce Help Documentation, "Validation Rule Formulas": This document outlines the functions available in validation rules. It explicitly lists PRIORVALUE(field) and describes its purpose: "Returns the previous value of a field." This confirms its suitability for the scenario.

3. Salesforce Trailhead, "Formulas and Validations" Module, "Use Validation Rules" Unit: This unit provides practical examples. A common example similar to the question is preventing a case status from reverting to a previous value, which uses ISCHANGED in conjunction with PRIORVALUE to check the state transition, demonstrating the core capability of PRIORVALUE to access the field's former state.