2D Low Energy Tubes are common lamps which are frequently found in corridors stairwells and the like.
Also called butterfly lamps by some people they have a reputation for reliability and long life.
They are also frequently used for emergency lighting.
They must not be used with presence detectors or PIRs as frequent switching ruins their life, they like to be left on for long periods.
They all Connect using small brass pins at the back of the middle section the smaller 2Ds usually have 2 pins the bigger ones 4 pins.
So with your old lamp in hand first choose the number of pins in the drop down box, then you will be asked the wattage, it will say on the old lamp but if in doubt the 16 watt is just about tea plate size, the 28 watt is dinner plate size the others which are less common a bigger still.
The colour is normally white but can be warm white. On your old lamp look for a number like 2700k or 3000k the lower the number the warmer the white, if you like crisp white light then go for the white