Option (iv) is the correct classification. Option (i) is incorrect because the context explicitly states that smoke is a heterogeneous mixture, not homogeneous. Option (ii) is incorrect because the context identifies vinegar as a homogeneous mixture, and muddy water (like sand and water) is heterogeneous. Option (iii) is incorrect because milk is a heterogeneous mixture (colloid). Option (iv) correctly identifies muddy water, milk, and blood as heterogeneous (blood has plasma and other components), and brass as homogeneous.