Each time the value of n increases by 1, the value of an increases by a factor of 3. The 3rd choice is appropriate.
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When an expression like this one is written in plain text, it is appropriate (required) to put parentheses around the exponent that contains an arithmetic operation.
an = 4·3^(n-1)
If you leave off the parentheses, the expression no longer describes a geometric sequence.
