The word for a god in Arabic is ilaah إله .
Rabb رب on the other hand means a lord. when you add ال to it = Ar-Rabb الرب then it refers to Allah as The Lord. otherwise, you can use the word Rabb in other ways. like رب الأُسْرَة Rabb-ul- Usrah lord of the family. Meaning the patriarch of the family, the one in charge and who takes care of it.
The same does NOT apply to ilah إله and Allah.
when you add ال to إله = al-ilah الإله the god or The God, depending on who uses it and what they refer to.
Whereas الله only refers to the One True God. it cannot be used for any other being. the great majority of linguists agree that it has no root word, it is not a derivative, nor can you add prefixes or suffixes to it. so you can't make it plural, feminine etc
but we can trace إله back to وَلَهَ walaha
in your example the question was من ربكم؟ who is your Lord? so you introduced the concept of Rabb in the question so naturally the answer has to be rabbuna ....... our Lord is...... then the answer is given الله
just llke if you were asked كم عمرك؟ kam 'umruk? How old are you? to give a full answer you'd say عمري كذا 'umri kadha. may age is such-and-such.
Hope that helps. بالتوفيق BitTwafeeq