Brass is a homogeneous mixture of 70% copper and 30% zinc by mass. Let's break down the answer and explain it in detail.

Homogeneous Mixture:
A homogeneous mixture is a mixture where the components are uniformly distributed throughout the mixture. In other words, the composition of the mixture is the same throughout. Brass is an example of a homogeneous mixture because the copper and zinc atoms are evenly distributed within the material.

Composition of Brass:
Brass is primarily composed of copper (Cu) and zinc (Zn). The composition of brass can vary depending on the desired properties, but the most common composition is 70% copper and 30% zinc by mass. This means that for every 100 grams of brass, 70 grams will be copper and 30 grams will be zinc.

Explanation of Option A:
Option A states that brass is a homogeneous mixture of 70% copper and 30% zinc by mass. This is the correct answer because it accurately describes the composition of brass. The high percentage of copper gives brass its characteristic reddish-brown color, while the zinc provides strength and corrosion resistance.

Explanation of Other Options:
Option B states that brass is a homogeneous mixture of 50% copper and 50% zinc by mass. This is not correct because the most common composition of brass is 70% copper and 30% zinc, not equal proportions of copper and zinc.

Option C states that brass is a homogeneous mixture of 70% copper and 30% tin by mass. This is incorrect because tin (Sn) is not a common component of brass. Tin is used in other alloys, such as bronze.

Option D states that brass is a homogeneous mixture of 50% copper and 50% tin by mass. This is also incorrect for the same reason as option C. Tin is not a common component of brass.

In summary, brass is a homogeneous mixture primarily composed of 70% copper and 30% zinc by mass. This composition gives brass its unique properties and characteristics.

Option a is correct