In scientific notation, when expressing a number in the form \( a \times 10^n \), the value of \( a \) must be between 1 and 10. Therefore, based on the options provided, the correct completion of the statement "a must be between 1 and ___" is:
10
However, since the options you provided do not include "10," if you are looking for a numerical response, it seems the options might be incorrect or misinterpreted. If you have specific options to choose from, please clarify so I can assist better!