Given: Volume of P = $x$; Volume of Q = $y$; Mass of P = $m$; Mass of Q = $n$.

It is given that the mass of Q is twice the mass of P, i.e., $n = 2m$.

Also, each side of Q is twice that of P. Let the side of P be ‘a’ units and the side of Q be ‘2a’ units.

So, the volume of P is: $x = a^3$, and the volume of Q is: $y = (2a)^3 = 8a^3$.

According to the question, $u = m/x = m/a^3$ … (1), and $v = n/y = 2m/8a^3 = m/4a^3$ … (2).

From equations (1) and (2), we get: $u/v = 4$, or $u = 4v$.

Hence, the correct answer is option (A).