Explanation
According to the PMBOK Guide, a project manager should monitor and control the project work by comparing the actual performance with the planned performance and analyzing the variances. If the budget at completion has shifted significantly and is higher than expected, it indicates that there is a cost overrun or a negative cost variance. The project manager should perform a root cause analysis of the project performance to identify the factors that caused the deviation and take corrective or preventive actions to bring the project back on track. Adjusting the original budget estimates, reviewing the activity duration, or negotiating the project changes are not the best options, as they do not address the underlying causes of the problem and may lead to further issues in the future. References: PMBOK Guide, 7th edition, page 205-206.