If 0° < θ < 90°, is sin θ equal to cos (90° - θ), and for what reason?
No, because angles measuring θ and 90° - θ are supplements.
No, because angles measuring θ and 90° - θ are complements.
Yes, because angles measuring θ and 90° - θ are supplements.
Yes, because angles measuring θ and 90° - θ are complements.

Solution:

We know that

Two angles are complementary if the sum is 90° 

Using the formula 

cos ( A - B ) = cos ( A ) cos ( B ) + sin ( A ) sin ( B )

We can write it as

cos ( 90° - θ ) = cos ( 90° ) cos ( θ ) + sin ( 90° ) sin ( θ )

As cos 90° = 0 and sin 90° = 1

We get

cos (90° - θ) = sin (θ) 

Therefore, angles measuring θ and 90° - θ are complements.

If 0° < θ < 90°, is sin θ equal to cos (90° - θ), and for what reason?
No, because angles measuring θ and 90° - θ are supplements.
No, because angles measuring θ and 90° - θ are complements.
Yes, because angles measuring θ and 90° - θ are supplements.
Yes, because angles measuring θ and 90° - θ are complements.

Summary:

If 0° < θ < 90°, sin θ is equal to cos (90° - θ), because angles measuring θ and 90° - θ are complements.