The correct answer is: C. $$\frac{{{\text{Standard hours for actual production}}}}{{{\text{Actual hours worked}}}} \times 100$$
Activity ratio is a measure of how efficiently a company uses its resources. It is calculated by dividing the standard hours for actual production by the actual hours worked. A high activity ratio indicates that the company is using its resources efficiently, while a low activity ratio indicates that the company is not using its resources efficiently.
Option A is the number of actual working days in a period divided by the number of working days in the budget period. This is not a measure of efficiency, as it does not take into account the number of hours worked.
Option B is the actual hours worked divided by the budgeted hours. This is a measure of efficiency, but it does not take into account the standard hours for actual production.
Option D is the standard hours for actual production divided by the budgeted standard hours. This is a measure of efficiency, as it takes into account both the actual hours worked and the standard hours for actual production. However, it is not a common measure of activity ratio.
Therefore, the correct answer is C.