Hearing, sight, sound, and smell are examples of that you can use to make observations (apex fill in the blank.)

Senses

Explanation:

There are total five senses in human being –  

a)Sight

b)Taste

c)Hearing

d)Smell and  

e)Touch

These senses help us to perceive the worldly things in a better way. For example – If taste buds would not have been there, then we would have never come to know the varieties of taste such as sweet, bitter, sour etc.  

In the same way we would have been never able to visualize the world as it is in actual if eyes (sight) were not there.

the answer is sex-linked recessive.