Since Paragraph 5A, the Delivery of Earnest Money and Option Fee Paragraph, uses the word within when describing the time period, Day One of the option period is the day after the effective date of the contract.
For example, if your client’s effective date is January 22 with a 10-day option period, the option period will end on February 1. If your client wishes to terminate under Paragraph 5B, the Termination Option Paragraph, she must provide notice to the seller by 5 p.m. local time where the property is located on February 1.