Q.What is atomic mass unit?

1 amu ≈ 1.66 × 10⁻²⁴ g (about the mass of a proton/neutron); option (A) as printed has a typo.

By definition, 1 atomic mass unit (amu, also written 'u') is exactly 1/12th the mass of one atom of the carbon-12 isotope:

$1\ \text{amu} = \frac{\text{mass of one }^{12}\text{C atom}}{12} = 1.66054 \times 10^{-24}\ \text{g} = 1.66054 \times 10^{-27}\ \text{kg}$

Looking at the printed options: option (A) is written as "1.66 × 10⁻⁴ g", but the coefficient 1.66 only matches the correct value if the exponent is 10⁻²⁴, not 10⁻⁴ — this looks like a printing/transcription error (a dropped digit in the exponent). Honestly flagging that typo rather than silently picking a literally-wrong power of ten, the next-closest printed option is (D) 1.67 × 10⁻²⁴ g, essentially the same order of magnitude and value as 1 amu (it i …