An atmosphere (atm) is a unit of pressure (standard atmospheric pressure).

Stones in the boat displace water equal to their weight; when thrown in, they displace only their volume, which is less, so the water level falls.

1 g/cm^3 equals 1000 kg/m^3, so the density of water is 10^3 kg·m^-3.

A light year measures the distance light travels in one year, so it is a unit of distance.

Energy change = T×Δ(area). For a bubble with two surfaces ΔA = 8π(4^2−2^2)=96π cm2; ΔE = 30×96π ≈ 9048 erg, so option D.

E = 2G(1+ν); with E/G = 2.4, then 2(1+ν)=2.4 so 1+ν=1.2 and ν=0.2.

The SI second is defined as 9,192,631,770 periods of the radiation corresponding to the transition between two hyperfine levels of the ground state of the cesium-133 atom.

The SI unit of energy in the metric system is the joule.

One US gallon is defined as approximately 3.785 litres, so 3.78 litres is the correct choice.

Atto (10^-18) is the smallest (least) prefix among the given options.