Final answer:
The stock must be sold regular way on or before Jan 18th to still receive the $0.90 dividend payable on Jan 30th, 2019.
Step-by-step explanation:
The earliest date that the stock can be sold regular way and still allow the customer to receive the $0.90 dividend payable on Jan 30th, 2019 is before the ex-date, which is Jan 18th. This is because the ex-date is the date on or after which a security is traded without a previously declared dividend. Therefore, if the stock is sold regular way on or before Jan 18th, the customer will still be eligible to receive the dividend.