what is the reason for adding tomato paste when making a brown stock?
It's odd that they force you to choose one, as many things in cooking have multiple reasons, e.g. browning meat adds flavour and colour to a stew. Are you sure the question isn't one of those 'tick all that apply' ones?

In this case, the tomato paste adds flavour, colour, and the acid helps break down the connective tissue in the bones, which helps the stock to jellify.