Which of the following is a dimensional constant?

If speed $V,$ area $A$ and force $F$ are chosen as fundamental units, then the dimension of Young's modulus will be :

Velocity $(v)$ and acceleration $(a)$ in two systems of units $1$ and $2$ are related as $V _{2}=\frac{ n }{ m ^{2}} v _{1}$ and $a_{2}=\frac{a_{1}}{m n}$ respectively. Here $m$ and $n$ are constants. The relations for distance and time in two systems respectively are

The physical quantities not having same dimensions are

$Assertion$ : Specific gravity of a fluid is a dimensionless quantity.

$Reason$ : It is the ratio of density of fluid to the density of water

If $P$ represents radiation pressure, $c$ represents speed of light and $Q$ represents radiation energy striking a unit area per second, then non-zero integers $x,\,y$ and $z$ such that ${P^x}{Q^y}{c^z}$ is dimensionless, are