Rum, rangpur or lime, ice, sugar. Recipe:

1. Wash the fruits and cut them into quarters, unpeeled.
2. Add citrus to a cup, with some sugar. Smash them together with a pestle or something similar, to squeeze the juice into the sugar.
3. Add ice and rum or vodka, in this order. Mix it well together.
4. Adjust sugar as desired, [optional] water it down a bit if too alcoholic for your taste.

Note: I think that the traditional version uses rangpur, but limes as in the pic are also extremely common. Some people (incl. myself) use vodka instead of rum.