The tracking signal is a measurement that indicates there may be bias in the forecast model. The tracking signal is the ratio of the cumulative forecast error to the mean absolute deviation (MAD). It measures how well the forecast is tracking the actual demand over time. A tracking signal of zero means that the forecast is perfectly accurate. A tracking signal within the range of -4 to +4 is considered acceptable. A tracking signal outside this range indicates that the forecast is consistently overestimating or underestimating the demand, which implies that there is bias in the forecast model. Bias is the tendency of a forecast to be consistently higher or lower than the actual demand. Bias can be caused by factors such as inaccurate data, inappropriate forecasting methods, or changes in demand patterns. References:
Managing Supply Chain Operations, Chapter 5: Demand Management and Forecasting, Section 5.2:
Forecasting Methods, Subsection 5.2.3: Forecast Accuracy and Control
CPIM Exam Content Manual, Module 3: Demand, Section 3.2: Forecasting, Subsection 3.2.2:
Forecasting Methods, Subsubsection 3.2.2.3: Forecast Accuracy and Control

Fixed order quantity is a method that places a replenishment order when the quantity on hand falls below a predetermined level, called the reorder point. The reorder point is calculated based on the expected demand during the lead time and the safety stock. The order quantity is fixed and constant, and it is determined by the economic order quantity (EOQ)formula or other criteria. Fixed order quantity is also known as the order point/order quantity (OP/OQ) system or the continuous review system.
Option A is not correct, because min-max system is a method that places a replenishment order when the quantity on hand falls below a minimum level, called the order point. The order quantity is variable and depends on the difference between the maximum level and the current inventory level. Min-max system is also known as the two-bin system or the bin system.
Option C is not correct, because periodic review is a method that places a replenishment order at fixed intervals of time, regardless of the quantity on hand. The order quantity is variable and depends on the difference between the target inventory level and the current inventory level. Periodic review is also known as the fixed order interval (FOI) system or the periodic order quantity (POQ) system.
Option D is not correct, because available-to-promise (ATP) is not a method of inventory replenishment, but a calculation of the uncommitted portion of the current inventory and planned production. ATP is used to promise delivery dates to customers based on the availability of inventory and production capacity.
References:
Inventory Management and Control
Inventory Replenishment Methods
Inventory Replenishment Policies