Can transactions scale sufficiently to compensate the diminishing block reward by fees?
Or a combination of both. It looks like things like the Lightning Network will be there for micro payments and smaller amounts so that only larger transactions will be directly on the blockchain. For larger (and aggregate) transactions a bit more fee shouldn't be a problem (a). Also at some point the block size will likely be raised allowing for more transactions per block as well (b).

Either way your title is wrong: there is no post-mining world.