Basically: when you start the game, your lifepod comes with 6 items; 2 filtered water bottles, 2 nutrient blocks, and 2 flares. If you could add one single item to that container when you start, what would you add?
P.S, if the item you chose has 2 or more parts, e.g the radiation suit, which gives a helmet, suit, and gloves, it will give you all 3 of those items.
I would probably pick the Water Filtration Suit as it would be very good early game.