Final answer:
When writing a letter to President Obama, address him as 'President Obama' and use the appropriate title. Avoid using 'Mr. Obama' or 'Barak Obama.' If unsure about someone's marital status, use 'Ms.'
Step-by-step explanation:
When writing a letter to President Obama, it is important to address him by his title as 'President Obama.' You should avoid using 'Mr. Obama' or 'Barak Obama.' If you are unsure about a person's marital status, it is appropriate to address them as 'Ms.' If a person holds multiple titles, use the highest ranking title they have. For example, you can address a person with a PhD and also a Professor as either 'Dr. John Doe' or 'John Doe, PhD.'