Place the decimal point right after the first nonzero digit, which is 3.
So you'll go from this
to this 
after this move.
Now ask yourself: "how many spots must I move the decimal point to get back to 0.003045?"
The answer is "3 spots to the left"
The exponent of the final answer is -3
The negative indicates "move left" and the 3 tells us how many places to move the decimal
Therefore,
Answer: Choice C)
This is assuming you meant to write the -3 as an exponent
So you'll go from this
Now ask yourself: "how many spots must I move the decimal point to get back to 0.003045?"
The answer is "3 spots to the left"
The exponent of the final answer is -3
The negative indicates "move left" and the 3 tells us how many places to move the decimal
Therefore,
Answer: Choice C)
This is assuming you meant to write the -3 as an exponent