Explanation:

Brass is an alloy composed primarily of copper and zinc. It is widely used in various applications due to its unique properties. Let's analyze each statement to determine its accuracy.

a) Brass is costlier than copper:
This statement is incorrect. Copper is generally more expensive than brass. Copper is a pure metal, while brass is an alloy made by combining copper with other elements such as zinc. The addition of zinc makes brass more economical compared to pure copper.

b) Brass has excellent corrosion resistance:
This statement is true. Brass is known for its excellent corrosion resistance properties. It is resistant to atmospheric corrosion, water corrosion, and many chemicals. The addition of zinc to copper forms a protective layer on the surface of brass, which helps prevent corrosion and extends its lifespan.

c) Brass has good machinability:
This statement is true. Brass is considered one of the most easily machinable metals. It has excellent chip formation, which makes it easier to cut, shape, and form into various components. Brass also has good dimensional stability, allowing for precise machining operations.

d) Brass has poor thermal conductivity:
This statement is false. Brass actually has relatively good thermal conductivity compared to many other metals. It conducts heat better than materials such as stainless steel or aluminum. However, it has lower thermal conductivity compared to pure copper.

In conclusion, the correct statements are:

- Brass has excellent corrosion resistance.
- Brass has good machinability.

Brass is a versatile material that is widely used in various industries, including plumbing, electrical, automotive, and decorative applications, due to its desirable properties such as corrosion resistance, machinability, and affordability compared to pure copper.