For starters, remember that "the working pen is to the left of that one that should have the green cap."
There are three possible arrangements for which this condition can be true:
? Green ? ?, ? ? Green ?, or ? ? ? Green.
UK Version
First, remember the condition: "The usable pen is to the left of the one with the green cap."
From this condition, you know that there are three possibilities for the correct line-up of caps: ?-green-?-?, ?-?-green-? and ?-?-?-green.
US Version
Next, remember that "when assembled correctly, all four pens have one white section."
The only pen without a white section currently is C, so you know that that white cap definitely belongs to pen C.
UK Version
Next, recall that "all pens will have one white part". Since the only pen currently without a white part is C, pen C definitely gets the white cap.
US Version
From Hints One and Two, you know that the correct order of caps can be either Blue, Green, White, Blue or Blue, Blue, White, Green.
You can now figure out the correct order from the puzzle's first clue.
UK Version
From Hints 1 and 2, you know that the order of the caps must be either blue-green-white-blue or blue-blue-white-green.
You can work out from the remaining condition what the correct order is.
US Version
Blue, Green, White, Blue can't be the correct order of the caps, becuse pen D would have the same color cap that it started with. This would violate the puzzle's first clue.
Therefore, the correct order of the caps must be Blue, Blue, White, Green.
That should give you the answer!
UK Version
The cap order blue-green-white-blue doesn't fulfil the first condition because pen D would have the same colour cap as it started with. The correct cap order must therefore be blue-blue-white-green.
Now you know the answer!
Solution
Incorrect
US Version
Too bad!
Go through each clue one at a time.
UK Version
Too bad.
Check the conditions one at a time.
Correct
Inkredible!
US Version
Pen C is correct.
UK Version
You can use pen C.
Once you pick them all up, however, it's easy to forget which one was usable.