The correct answer is: D. Are under burnt.
Second class bricks are under burnt bricks. They are not fully baked and have a high moisture content. This makes them weaker and less durable than first class bricks. Second class bricks are often used for non-load-bearing walls or as filler material.
A. Are of dark brown color: This is not a defining characteristic of second class bricks. Bricks can be of any color, depending on the type of clay used and the firing temperature.
B. Produce a metallic sound when struck: This is a characteristic of well-burnt bricks. When struck, a well-burnt brick will produce a clear, ringing sound. Under burnt bricks will produce a dull, thudding sound.
C. Are well burnt: This is the opposite of the correct answer. Second class bricks are under burnt, not well burnt.