To find the recursive formula for the given arithmetic sequence \(7, 5, 3, 1, -1, \ldots\), we first observe that each term decreases by 2 from the previous term.
The first term of the sequence is: \[ a_1 = 7 \]
The relationship between consecutive terms can be described as: \[ a_n = a_{n-1} - 2 \]
Therefore, the recursive formula for the sequence is: \[ \begin{align*} a_1 &= 7 \ a_n &= a_{n-1} - 2 \quad \text{for } n > 1 \end{align*} \]
Thus, the correct response is: \[ \text{a)} \quad a_1 = 7, \quad a_n = a_{n-1} - 2 \]