The kinetic energy of a body is increased most by doubling its:
(a) Speed
(b) Density
(c) Weight
(d) Mass

As known the formula for kinetic energy $K=\frac{1}{2}mv^2$

On doubling the mass, the kinetic energy becomes $K'=\frac{1}{2}\times2m\times v^2=2K$

On doubling the speed, the kinetic energy $K'=\frac{1}{2}m(2v)^2$

$=4K$

Thus on doubling the speed, the kinetic energy increases most.

Therefore, option (a) is correct.