What is the stall speed of A330 200?

The short answer is that at landing, for the A330 something in the neighborhood of 120 knots (220 km/h / 140 mph ) would be its stall speed, based on a landing speed of 135 knots. ROUGH NUMBERS.

The long answer is that stall speed of any aircraft varies with a bunch of physical quantities, including: Weight (no. of passengers, weight of cargo, amount of fuel on board), density of the air (temperature and atmospheric pressure, humidity?), aircraft configuration (flaps - how far are they extended, maybe even landing gear down or up) and even contamination of the wings - ICING (layer of ice built up on the wings and other surfaces).

All of this can be tested on a model or prototype and put on some complicated graphs and tables. In modern practice, the pilots use the FMC or FMS (Flight Management Computer / System) included in the cockpit or special software on a company laptop to input some of things mentioned above and get a number out for certain speeds, such as the landing speed.

Of course they could look in the graphs / tables and do it that way, and when these are put in an operating manual (I imagine as opposed to pure engineering documents), these can come with instructions on how to decide on the right speeds - it gets to be kinda like doing taxes, where 'if line 83 is less than 1.8 times line 54 please add line 5 to the number of donuts you had this morning' but it's all with safety in mind.

Btw all these speeds are relative to the air, not the ground - which is why a sudden change in the wind can bring an aircraft closer to stalling.

And in any aircraft, there will be something to yell at you when you get too close to the stall speed - this probably uses some other measurement, instead of looking at speed and figuring it out from there.