A "principal square root" is always ...

A: a negative fraction

B: a positive integer

C: a negative integer

D:a positive fraction

Really none of the above.

A principal square root of a real number is a positive real number (or a positive real number times i, if you're up to complex numbers, but let's assume not.) It's not necessarily a positive integer, √2 being the obvious example.

Is √2 a positive fraction? We can write it as √2 / 1 so I suppose it is. It's certainly not a rational number. Even if we grant irrationals as fractions surely √0=0 isn't a positive fraction. So "always" isn't correct.

I'd go with none of the above, but if I had to choose I'd choose D.