There is a negative correlation between the sales of sweaters and the sales of sandals.
Let us check each of the options.
A. Age of customers.
Customers of every age can wear sweater or sandals , so age is not a factor that affects the sales.
B. Price of Socks.
Price of socks cannot be too high as it is not a very costly material.
So price of socks is not a lurking variable that will affect the correlation.
C. Outside temperature.
If the weather outside is cold, people may not buy more sandals and its sales will get decreased. Also cold weather means sale of more sweaters by the customers. So this factor affects the sale of these two items.
So we can say Outside temperature will be the lurking variable that explains the correlation.
D. Number of Customers:
Number of customers does not depend on what item is selling .
So it cannot be lurking variable.