Correct spelling, explanation: senior is the correct spelling; it is a combination of Latin senex (old man) with the comparative suffix -or. Senoir is incorrect also due to its supposed pronunciation: it would be pronounced [se-nuh-wa:], not [seen-yer] as the correct form. These points show that senior is the only correct form.

Definition of senior:
noun, a person who is older than another person; the oldest class in school
My grandma attends English classes for seniors in our local community centre.
In our school, the gazette is led by Jack Kaminsky. He is a senior.
adjective, older than another in a family context (for example when a son gets his name after a father; the father is considered senior); with a higher position at work
Have you met senior Mr. Smith?
Jane is a senior accountant in her office.

Incorrect spelling, explanation: this form is incorrect; the correct form is senior which comes from the word senex (old man in Latin with the comparative adjective suffix -or. Also, the correct pronunciation is [seen-yer], while the form senoir would be pronounced [se-nuh-wa]. This is why senior is the correct form.

Incorrect spelling

Incorrect spelling

Incorrect spelling