What is the difference between a ribbed vault and a groin vault?
a. groin vaults are built on the sides, while rib vaults are on the ceiling.
b. groin vaults have more support than rib vaults.
c. rib vaults are groin vaults with extra stone ribbing.

The correct answer is C. Rib vaults are groin vaults with extra stone ribbing.

Explanation:

First let's establish what a barrel vault is.  A barrel vault is basically a tunnel, an extended round arch that creates a tunnel.  The perpendicular intersection of two barrel vaults is called a groin vault.  These vaults were used in Roman architecture.  

Sometimes groin vaults may be pointed instead of round; this means the barrel vaults are pointy and not fully round.  The intersection of these two pointed barrel vaults forms what it's called a rib vault. (See attached pictures to view differences)

Rib vaults were typically use in Gothic architecture.  They are more efficient regarding weight carriage as they spread the weight evenly through the stone ribs out and down the pillars on the ground and outside flying buttresses (see attached picture) instead of walls themselves, like groin vaults do.  Apart from the visual pointy feature, that is the main difference between the two types of vaults.  

The extra stone ribbing provides more support to; therefore the fillings are thinner making the construction lighter.  

In the Gothic period the constructions were meant to symbolize closeness to God and all his glory.  That is why height was important and everything was pointing up as it were to merge with the sky.  These ribbed stones helped release the weight from the walls and became the main structural supporting element of the ceiling, meaning more windows could be added to the walls, therefore more light and air, also used to symbolize the closeness to God.  

homophonic or a tonal