My thinking process: 

- a passes a block to b
- b passes the block to c
- it is sent #value and returns 1
- which is thrown away
- the return value becomes 3
- that becomes the return value of the first block in #b:
- a check is made to ensure 2
- what kind of test is ensure running?
- [3] ensure: [2]
- i'll say #ensure: is testing their is an object there
- so the answer is 3

(spoilers)
The actual answer: 

turns out the answer is aFoo, so I guess all numbers were a ruse and it returned self. 

Why, I couldn't say. I guess #ensure: has a property I don't understand. 

Chris