Under the feudal system, a higher noble granted land to a lower noble in exchange for: a. the use of the lower noble's serfs. b. a payment in the form of money or livestock. c. a portion of the lower noble's crop yield. d. an oath of loyalty.

Hey there budd!

This was actually known to be an exchange for an oath of loyalty. So, when the the feudal system, a higher noble granted land to a lower noble in exchange for an oath of loyalty which would signify giving for free, sense it would be known as being loyal and actually recognizing what it needed to be done.

Hope this helps you budd! 

D. an oath of loyalty.