Two electric bulbs marked 100W 220V and 200W 200V have tungsten filament of same length. Which of the two bulbs will have thicker filament?

We know, 

P = V²/R 

⇒More the power, lesser the resistance. 

The resistance of 200W bulb is lesser than the 100W bulb.

Also

R = ρ l/A

⇒Higher the resistance,lesser will be the value of A. Thus, thinner will be the wire. 

The wire of 200W bulb is thicker than 100W bulb, as it's resistance is lesser.

Basically, 

P ∝A