A. -i

B. i

C. -1

D. 1

1. Imaginary numbers are an extension of the real number system. The imaginary unit is denoted as [tex]\(i\)[/tex], where:

[tex]\[ i^2 = -1 \][/tex]

2. By definition, the square of [tex]\(i\)[/tex] gives us [tex]\(-1\)[/tex]:

[tex]\[ i^2 = -1 \][/tex]

3. Therefore, [tex]\(i\)[/tex] is defined as the principal square root of [tex]\(-1\)[/tex]:

[tex]\[ \sqrt{-1} = i \][/tex]

Therefore, the square root of [tex]\(-1\)[/tex] is [tex]\(i\)[/tex].

So, the correct answer is:

i