Answer: giga
Step-by-step explanation
"giga" is the prefix that means "billion".
For instance, "gigabyte" means "billion bytes" approximately (technically it's 1,073,741,824 bytes, but we are close enough by saying "1 billion bytes")
The term "nano" means "billionth". Something like "1 nanometer" means "1 billionth of a meter". Be careful not to mix up the terms "billion" and "billionth".
"deca" means "ten". Think "decade is 10 years" as one way to remember. Be careful not to mix up the prefix "deca" with "deci".
"mega" means "million". Both start with m, so that could be one way to remember.