Basically null values are considered when we dont know the actual value... so we can say that null values are not comparable..(arithmetic operation can't be performed on null as they are incomparable.. )
Now here in your question they have asked that which of the following is false... so (c) option will be better choice over here because eventually candidate key is minimal of primary key and primary keys doesn't not contain null values because it can cause loss of data .. as you are saying null is unique value we can't say that bcz its value is unknown so it may or may not be unique... so answer here will be option C.(Bcz passport number can not be CK as it contains null)