Explanation:

- To find the pattern, we look at the sequence: 1, 1, ?, 15, 105, 945.

- Notice the relationships:

- $1\to 1$: Unchanged, multiplied by 1.

- $1\to ?$: Need to determine the multiplier.

- $?\to 15$ is obtained by a certain multiplier.

- Multiply 15 by 7 to get 105.

- Multiply 105 by 9 to get 945.

- Assume the pattern involves multiplying by consecutive odd numbers: 1, 3, 5, 7, 9.

- The missing step is obtained by multiplying 1 by 3 to get 3.

- Correct Answer: Option 2, 3

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