You have chosen the correct answer and I’m not really sure I can explain why you’re right.
The number 12 has been part of the daily life of humans since the beginning of recorded history ... the most obvious use of the number 12 was in the separation of the celestial equator into the twelve houses of the zodiac ... from the zodiac we get the twelve months of the year.
All history books agree that human history began in what was called the fertile crescent ... Mesopotamia ... the land between the Tigris and the Euphrates rivers ... the Sumerians were the first to do almost everything that can be described as civilization or culture ... you name it ... they did it ... hybridization of plants ... mathematics ... astronomy ... fabrics ... metallurgy ... poetry ... a written language ... dance ... accounting ... you name it ... they did it.
Dividing the sky into constellations was one of their better ideas ... the Sumerians had a base 60 numbering system and the 360º circle fitted neatly into their mathematical models ... they established the 12 houses of the zodiac of 30º each ... the houses were actually constellations (groups of stars) and the sun, moon and planets were plotted in relation to the backdrop of the constellations ... this all happened circa 3800 BCE and from that time onward, the number 12 has been crucial to the pantheons of the subsequent civilizations.
One of the most convincing arguments for the sacredness of the number 12 can be found in our modern base 10 numbering system ... the numbers from 1 to 12 all have distinct names ... we don’t say eight, nine, ten, oneteen, twoteen ... however, when we get above 20 we begin to use twenty one, twenty two, thirty one, thirty two, etc. until we encounter the eleven and twelve again ... one hundred eleven, one hundred twelve, etc.
Regardless of how or why ... the number of Olympians remained constant at 12 ... when Herakles became immortal, he refused Zeus’ offer to become one of the Olympians because he said that it would be unfair to oust one of the current Olympians so that he could become one.
The 12 Olympians are:
Zeus
Hera
Athene
Demeter
Aphrodite
Poseidon
Dionysos
Apollon
Ares
Hermes
Hephaistos
Artemis