Answer:
A) 10
Explanation:
In the US, a number in scientific notation will have a mantissa (a) such that ...
1 ≤ a < 10
That is, the value of "a" must be between 1 and 10 (not including 10).
_____
Comment on alternatives
In other places or in particular applications (some computer programming languages), the standard form of the number may be a×10^n with ...
0.1 ≤ a < 1
In engineering use, the form of the number is often chosen so that "n" is a multiple of 3, and "a" is in the range ...
1 ≤ a < 1000
This makes it easier to identify and use the appropriate standard SI prefix: nano-, micro-, milli-, kilo-, mega-, giga-, and so on.