It's not entirely clear, in 3.5e, how counterspelling works. I only see the standard "cast on the defensive" and "Counterspell" sections in the PHB. Is it possible to counterspell on the defensive? Are there sources and references to support? Can you, in general, ready a defensive action?
The rules for counterspelling state that you need to cast the spell:
To complete the action, you must then cast the correct spell.
This is no different than casting a spell normally, and follows all the standard rules for spellcasting - it still provokes an Attack of Opportunity unless you Cast Defensively.
To complete the [counterspell], you must then cast the correct spell.
You’re casting the spell, with everything that entails: you provoke, but also have the option of casting defensively. It is in every way identical to casting the spell regularly, excepting only that 1. you had to ready an action to get the timing right, and 2. instead of getting the spell’s effect, you counter the foe’s.
There is no rule that comes out and explicitly confirms that casting defensively is an option here, but there doesn’t need to be, since the rule is that you are casting, and no further rules limit that.
Either way, please be aware that counterspelling is a really low-value tactic. Even with a lot of investment (e.g. Divine Defiance so you can counterspell as an immediate action, Inquisition Domain for that nice +4 bonus on the dispel magic check, etc.), it tends to result in a lot of wasted actions. And if you aren’t going for immediate-action counterspells, and have to ready an action anyway, counter-nuking (readying a nuke spell instead of dispel magic, to deal damage and then force an impossible Concentration check to save the spell) is a far more effective strategy.