Let us write the complex number $$z$$ in its Cartesian form
$$z = x + i\,y,\qquad x,\;y \in \mathbb R.$$
First compute the numerator
$$i z - 2 \;=\; i(x + i\,y) \;-\; 2
\;=\; i x \;-\; y \;-\; 2
\;=\; (-\,y - 2) \;+\; i\,x.$$
Next compute the denominator
$$z - i \;=\; x + i\,y - i
\;=\; x \;+\; i\,(y - 1).$$
So the required fraction becomes
$$\frac{i z - 2}{z - i}
\;=\;
\frac{(-\,y - 2) + i\,x}{x + i\,(y - 1)}.$$
To extract its imaginary part, multiply numerator and denominator by the conjugate of the denominator:
$$\frac{(-\,y - 2) + i\,x}{x + i\,(y - 1)}\;
\cdot\;
\frac{x - i\,(y - 1)}{x - i\,(y - 1)}
=
\frac{\bigl[(-\,y - 2) + i\,x\bigr]\,
\bigl[x - i\,(y - 1)\bigr]}
{x^{2} + (y - 1)^{2}}.$$
Expand the numerator term-by-term:
$$\bigl[(-\,y - 2) + i\,x\bigr]\,
\bigl[x - i\,(y - 1)\bigr]$$
$$=\;(-\,y - 2)\,x
\;+\;
(-\,y - 2)\,[-\,i\,(y - 1)]
\;+\;
i\,x\cdot x
\;+\;
i\,x\cdot[-\,i\,(y - 1)].$$
Calculate every product:
$$(-\,y - 2)\,x
\;=\;
-x\,y - 2x,$$
$$(-\,y - 2)\,[-\,i\,(y - 1)]
\;=\;
(+)\,i\,(y + 2)(y - 1),$$
$$i\,x\cdot x
\;=\;
i\,x^{2},$$
$$i\,x\cdot[-\,i\,(y - 1)]
\;=\;
+\,x\,(y - 1).$$
Gather the real and imaginary parts:
Real part:
$$-x\,y - 2x
\;+\;
x\,(y - 1)
\;=\;
(-x\,y + x\,y)
- 2x - x
\;=\;
-3x.$$
Imaginary part:
$$i\Bigl[(y + 2)(y - 1) + x^{2}\Bigr].$$
Therefore
$$\frac{i z - 2}{z - i}
\;=\;
\frac{-\,3x
\;+\;
i\left[x^{2} + (y + 2)(y - 1)\right]}
{x^{2} + (y - 1)^{2}}.$$
The denominator is purely real, hence
$$\operatorname{Im}\!\left(
\frac{i z - 2}{z - i}
\right)
\;=\;
\frac{x^{2} + (y + 2)(y - 1)}
{x^{2} + (y - 1)^{2}}.$$
The given condition is
$$\operatorname{Im}\!\left(
\frac{i z - 2}{z - i}
\right)
+ 1
= 0
\;\;\Longrightarrow\;\;
\operatorname{Im}\!\left(
\frac{i z - 2}{z - i}
\right)
= -1.$$
Hence
$$\frac{x^{2} + (y + 2)(y - 1)}
{x^{2} + (y - 1)^{2}}
= -1.$$
Cross-multiply:
$$x^{2} + (y + 2)(y - 1)
= -\bigl[x^{2} + (y - 1)^{2}\bigr].$$
Bring every term to the left side:
$$x^{2} + (y + 2)(y - 1)
+ x^{2} + (y - 1)^{2}
= 0.$$
Combine like terms:
$$2x^{2}
+ \bigl[(y + 2)(y - 1) + (y - 1)^{2}\bigr]
= 0.$$
Compute the brackets one by one:
$$(y + 2)(y - 1)
= y^{2} + y - 2,$$
$$(y - 1)^{2}
= y^{2} - 2y + 1.$$
Add them:
$$y^{2} + y - 2
+ y^{2} - 2y + 1
= 2y^{2} - y - 1.$$
Hence
$$2x^{2} + 2y^{2} - y - 1 = 0.$$
Divide by 2 to simplify:
$$x^{2} + y^{2} - \frac{y}{2} - \frac{1}{2} = 0.$$
Move the constant to the right side:
$$x^{2} + y^{2} - \frac{y}{2}
= \frac{1}{2}.$$
To obtain the standard circle form, complete the square in $$y$$:
$$y^{2} - \frac{y}{2}
= y^{2} - \frac{1}{2}y
= \left(y - \frac{1}{4}\right)^{2} - \frac{1}{16}.$$
Substitute this into the equation:
$$x^{2}
+ \Bigl[\bigl(y - \tfrac14\bigr)^{2} - \tfrac1{16}\Bigr]
= \frac12.$$
Rearrange:
$$x^{2} + \bigl(y - \tfrac14\bigr)^{2}
= \frac12 + \frac1{16}
= \frac{9}{16}.$$
Thus the locus is a circle with centre
$$\bigl(0,\; \tfrac14\bigr)$$
and radius
$$\sqrt{\frac{9}{16}}
= \frac34.$$
Consequently, the radius asked in the problem is $$\dfrac34$$.
Hence, the correct answer is Option C.