I think we need more information or assumptions? Terrorists are not arrested at random.
Perhaps there were only 83 terrorists in the 1000 population and all have been arrested. Then p = 0.
Perhaps all 1000 are terrorists and only 83 have been arrested so far. Then p = 1.
So, probably the answer is somewhere between 0 and 1.
There is no proportion presented for the ethnic group compared to the pool from which terrorists are drawn, beyond the simple assertion they are a "minority."
The Venn diagram given thus far is an unknown size population with two subsets and an overlap region. The overlap is stated as 83/100 terrorists belong to ethnic group, and so the remainder (17/100 terrorists) belong to the external slice of terrorists that are not members of the ethnic group.
But there is no specification of the proportions for ethnic group, as in what proportion of the ethnic group are also members of the terrorist group.