Of these choices, the word would be 'on'. "I play on the soccer team."

You can't use 'at' or 'inside'. These would refer to a place ( e.g. at/inside the stadium). 'At' could also refer to a time, for example. Of the other two options, both 'on' or 'in' are possible. American English uses 'on', while in British English you can say either 'play in a team' or 'play on a team'. The obvious clue is the word 'soccer', which is the word AmE speakers use to refer to the game which most other nations call football/ футбол / voetbal / fútbol / futball / fotboll /Fussball / फ़ुटबॉल / フットボール . I'm guessing this question was written by an AmE speaker who doesn't realise that there are two possible answers. Just keep things simple and choose 'on'. You can be sure that 'on' will be marked right.