Question

Which of the following is used as SI prefixes:
A. Femto
B. Atto
B. Zepto
D. All of the above

Answer 1

SI units are standard internationally accepted units. They are also referred to as MKS units where M stands for meter as SI unit for length, K stands for kilograms, unit for mass and S stands for seconds, unit for time.

Complete answer:
Prefix is something that is attached before a word to make another meaningful word. Like meter is a word and if we add centi in the beginning (which means 100), we get a word centimeter. Now, it is not a coincidence that 100 cm make up a meter. In a system of units, when a prefix is added to a unit, it means that some order of magnitude is concerned like in centimeters the order was -2 ( 10 to the power -2).
Well, we should go through all the options one by one to know the correct answer.
Option A: femto
It is very commonly used at nuclear scales. Radius of a nucleus is in the femtometer range. It can be written as
1 fm = $10^{-15}$ m.
where fm is notation used to denote femto meter.
Similarly,
Option B: Atto is also used as a prefix to SI system units,
1 am = $10^{-18}$ m.
And,
Option C: zepto is also a prefix so that
1 zm = $10^{-21}$ m.
Which leaves us with the last option that is, all of the above options A, B and C are correct.

So, the correct answer is “Option D”.

Note:
The shorthand notations that we used here like fm, am and zm are standardized accepted notations. It happens that if we have positive powers in the order of magnitude, we write capital letters and if we have negative powers, we use small letters in shorthand prefixes. For example, Mega is a prefix which contributes 10$^3$ to any unit where it adds like MHz is a unit of frequency.