Since lower case m stands for both the prefix. milli and for unit for length meter, how do we tell which one is meant when they are used?
a: a prefix is never used alone. so a lone m (lower case) would refer to a unit, the meter (the si metric unit of length.)
b: the prefix comes before the unit, so mm (both lower case) would stand for millimeter.
c: the prefix comes before the unit, so mg (both lower case) would stand for milligram.
d: all of the other 3 answer choices.

D: all of the other 3 answer choices

Explanation:

Milli- is a prefix, so a lowercase m in front of any other letter means milli-.  For example, mm = millimeter and mg = milligram.

An m by itself means meters.

D: all of the other 3 answer choices.

Explanation:

A: a prefix is never used alone. So a lone m (lower case) would refer to a unit, the meter (the SI metric unit of length.)

Above statement is true because here we can see that when prefix is placed before any SI unit then only we use it let say if we use only unit "m" then it is clearly assumed to be meter

B: the prefix Comes before the unit, so mm (both lower case) would stand for millimeter.

TRUE

Since prefix is always used before SI unit then we have to use the prefix like it is shown here for length. when we give unit as "mm" then it will show millimeter.

C: the prefix comes before the unit, so mg (both lower case) would stand for milligram.

TRUE

Since prefix is always used before SI unit then we have to use the prefix like it is shown here for mass. when we give unit as "mg" then it will show milligram.

a.   mass number from atomic number