I was asked this question when I was doing my gap year of Newfrontiers training, twelve years ago. The three options I was given were these:

 
Twelve years ago, I was confused, so I put up my hand for all three of them. I had a feeling it was probably #1, but the teacher at the time believed it was #3. Then, several years later, I became persuaded (as I still am) that in fact it’s #2.
 
The main reason for saying this is the immediate context. Forget the chapter break for a moment, and an important point emerges:

So then you are no longer strangers and aliens, but you are fellow citizens with the saints and members of the household of God, built on the foundation of the apostles and prophets, Christ Jesus himself being the cornerstone, in whom the whole structure, being joined together, grows into a holy temple in the Lord. In him you also are being built together into a dwelling place for God by the Spirit. For this reason I, Paul, a prisoner for Christ Jesus on behalf of you Gentiles—assuming that you have heard of the stewardship of God’s grace that was given to me for you, how the mystery was made known to me by revelation, as I have written briefly. When you read this, you can perceive my insight into the mystery of Christ, which was not made known to the sons of men in other generations as it has now been revealed to his holy apostles and prophets by the Spirit. This mystery is that the Gentiles are fellow heirs, members of the same body, and partakers of the promise in Christ Jesus through the gospel. (Eph 2:19–3:6)

 
I think it’s pretty hard to argue, given the flow of thought, that these two mentions of the “apostles and prophets” refer to different things. And in 3:5, Paul seems to be talking about #2: the new covenant apostles and prophets of the first generation, to whom the mystery of the inclusion of the Gentiles has now been revealed, as it hadn’t been previously. That would probably mean he is also talking about this in 2:20 – the united church of God, both Jew and Gentile, is built on the foundation of revelation given to the apostles and prophets, in the first generation of the church, about God’s worldwide purposes.
 
Do you agree?