I can give you the answer but I think it is more important that you understand the question and the options.
In the question, we state that the decision strategy is defined on the Customer class. This is the typical place where you define strategies. So not on the work object class! This means the 'Apply to' class of the strategy is Customer. Therefore in the strategy you have access to Customer properties from this page using the Primary or better the Customer (class label) keyword. For e.g. Customer.Age, Customer.Income etc.
Also, you might remember, that in a decision strategy one has always access to proposition properties and strategy (temporary) properties using the dot operator. E.g. .Price, .Cost etc. All these properties are defined on what we call an SR class (strategy result).