If n is a term of the sequence 2, 5, 8, 11, ..., which expression would give the value of n? A) 2n + 1 b) 2n - 1 c) 3n - 1 d) n + 3

If k is the index of the term of the series whose value is n, then we have
  n = 3k - 1
suggesting that C is the answer of choice.
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(3n-1) does not give the value of n. It gives the value of the n^th term of the series.
For n=1, the 1st series term is 3*1-1 = 3-1 = 2
For n=2, the 2nd series term is 3*2-1 = 6-1 = 5
For n=3, the 3rd series term is 3*3-1 = 9 -1 = 8
...

(It appears that several of the questions on this quiz are badly worded. This is one of them.)

n+3 (answer D) will give the value of the next term.