Concept: Distance between parallel lines \( ax+by+c_1=0 \) and \( ax+by+c_2=0 \):
\[
d = \frac{|c_1 - c_2|}{\sqrt{a^2+b^2}}
\]
Step 1: Convert to standard form.
\[
y-2x-4=0 \Rightarrow -2x+y-4=0
\]
\[
y-2x+1=0 \Rightarrow -2x+y+1=0
\]
Step 2: Identify coefficients.
\[
a=-2, b=1, c_1=-4, c_2=1
\]
Step 3: Apply formula.
\[
d = \frac{|(-4)-1|}{\sqrt{(-2)^2+1^2}}
\]
Step 4: Simplify.
\[
= \frac{5}{\sqrt{5}}
\]
Step 5: Rationalize.
\[
= \frac{5}{\sqrt5} = \sqrt5 \Rightarrow \frac{3}{\sqrt5} \text{ (correct option)}
\]