Which of the following should be the FIRST sentence after the rearrangment ?

The sentences after correct arrangement would read as:
(The sentence number in question has been left unchanged for ease of understanding.)

5. One day, Zeus, the king of Mount Olympus, was giving out gifts to beasts, birds and insects. (There is introduction of an event and it's characters. Hence, this will be first sentence of the para.)

1.To his surprise, a little honeybee came before his throne and said, "Of all the gifts you could give me, only one will do. I' d like the power to inflict great pain." (This will naturally form the second sentence of the para as it is in sync with first sentence.)

3."What an awful wish !" said great Zeus, "But I will grant it". (This follows as here Zeus replies.)

2. I hereby give you a sharp sting. But, I am sure you will use this weapon carefully only in times of anger and strife. (Zeus gives the gift what bee has asked for.)

6. "You will get to use it only once, for using it will cost you your life." (Zeus,here, continues adding bane to the boon)

4. And to this day, the little honeybee dies after it stings. (This will naturally constitute the end of the paragraph.)

Hence, the correct option is A.