10.20.1.0/24 is a BD in ACI because of the part which says "direct, pervasive". For the other answer choice, we see form the administrative distance and the "internal" naming, we know that this is an external subnet (172.16.100.0/24) and that as all external routes are exchanged withing the fabric through MP-BG. MP-BGP is actually used to populate the leaves in the fabric with routing on how to reach those external routes. In MP-BGP, the spines are only the route reflectors. So from this we know that the next hop of the external subnet will be a VTEP of a leaf.