Due to parallax error, the starting and ending points are 0.5 cm and 4.2 cm, respectively.
Therefore the observed length is,
Observed Length = 4.2 - 0.5 = 3.7 cm = 37 mm (since 1 cm = 10 mm)
On the other hand, when measured correctly, the starting and ending points are 1 cm and 4 cm, respectively.
Therefore the actual length is,
Actual Length = 4 - 1 = 3 cm = 30 mm (since 1 cm = 10 mm)
That’s why we should always view from the angle that leads perpendicular to the marking on the scale.
Hence, the correct answer is option C.