Yes I've noticed this. Unfortunately it's up to the user (Asker) to report an answer incorrect.
Answer:
see explaination
Explanation:
If I am to select I will select the strategy Impute the missing values . Because rather than deleting and all the other strategies. This is the best option because, this is an optional field as described there is no need for such accurate information about salary.
If we want to maintain this field we can just fill the value by identifying the similar records which is the other customers who is visiting the same no of times and fill that.
LEFT SIDE :
Layer menu
Layer filters
Layer groups
Link layers
BOTTOM :
Layer effects
Layer menu
Visibility
RIGHT SIDE :
Add new layer
Opacity
Fill opacity