How do I understand this gear diagram?

[Navier-Stokes]

I'm finding it a bit too confusing to imagine how it actually would look like. But once I understand that, I'd easily be able to solve the problem!

link: http://i.imgur.com/DApJzGx.jpg

The attempt at a solution:
I'm thinking that the dotted line might be the place where the gears mesh, however that assumption isn't helping me to make any advances.

 

[haruspex]

I'm fairly confused too.
I would say that in 1 and 4 only the top halfof the gear is shown, whereas for 2 and 3 the whole gear is shown. 1 and 2 mesh at the parallel horizontal lines, likewise 3 and 4.
From the diagram, I would have said 1 and 4 were connected by being on a common shaft, but that does not make sense given the other information. So I have to guess that 2 and 3 are supposed to be on a common shaft.
There does not seem to be enough information to answer either a or b in isolation, but I see only one combination of answers that works.

 

[Nidum]

Gears 1 and 4 are on a common axis but can rotate independently .

There is to be a shaft between gears 2 and 3 . Correct meshing of all gears is only possible if the number of teeth on gear four is chosen so that the centre distances of gear pairs 1 to 2 and 3 to 4 are the same .

 

[Dr.D]

It certainly looks like a shaft was omitted between gear 2 and gear 3. If this is included, then the result is an epicyclic train. Correct meshing is assured because we are given that all gears have a common module. The dotted line is nothing more than a center line, not a physical shaft.