Step 1: Concept The equation represents two parallel lines if the second-degree part is a perfect square. Distance $d = \frac{2\sqrt{g^2 - ac}}{\sqrt{a(a+b)}}$ or $d = \frac{|c_1 - c_2|}{\sqrt{a^2 + b^2}}$.
Step 2: Meaning $4x^2 + 4xy + y^2 = (2x+y)^2$. The equation is $(2x+y)^2 - 3(2x+y) - 4 = 0$.
Step 3: Analysis Let $t = 2x+y$. Then $t^2 - 3t - 4 = 0 \implies (t-4)(t+1) = 0$.
The lines are $2x+y-4=0$ and $2x+y+1=0$.
Distance $d = \frac{|-4 - 1|}{\sqrt{2^2 + 1^2}} = \frac{5}{\sqrt{5}} = \sqrt{5}$.
Step 4: Conclusion The distance between the lines is $\sqrt{5}$ units.
Final Answer: (C)