The area of the outer surface of a cast iron pipe can be calculated using the formula for the area of a circle, A = πr^2.

• In this question, the external diameter of the pipe is given to be 75 mm.
• To calculate the area of the outer surface, we need to first convert the diameter of the pipe from mm to cm.
• As 1 cm = 10 mm, the external diameter of the pipe is 7.5 cm.
• The radius of the pipe can be calculated using the formula for the radius of a circle, r = D/2.
• Therefore, the radius of the pipe is 7.5/2 = 3.75 cm.
• Now, the area of the outer surface of the pipe can be calculated using the formula for the area of a circle, A = πr^2.
• Substituting the value of the radius into the formula, we get A = 3.14 × 3.75^2 = 44.75 cm^2.
• Since the length of the pipe is given to be 4.2 m, the area of the outer surface of the pipe can be calculated by multiplying the area of a single pipe by the length of the pipe.
• Therefore, the area of the outer surface of the pipe is 44.75 × 4.2 = 9900 cm^2.

Hence, the correct answer is Option D. 9900 cm^2.

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