If the sin 90 degrees is 1, then the =

We'll use this identity: 
cos(90 - x) = sin(x)
where x is any real number

If x = 90, then sin(x) = sin(90) = 1 which is given to us. 

So this means

cos(90 - x) = sin(x)
cos(90 - 90) = sin(90)
cos(0) = 1

So it's likely your teacher is looking for you to write cos(0) = 1

However, the prompt and instructions are a bit vague. There are other possibilities that I may not be thinking of. 

Cis your correct answer