a) State Newtons Second Law of motion. Hence derive F = ma from it.
b) A body is moving along a circular path such that its speed always remains constant should there be a force acting on the body?

a) Newtons Second Law of Motion :
The rate of change of momentum of a body is directly proportional to the external force acting on the body.
Derivation of F = ma
According to Newtons second law
Force $\propto$ Rate of change of Momentum
$F\propto \frac{\mathrm{dp}}{\mathrm{dt}};\mathrm{F}=\mathrm{k}\frac{\mathrm{dp}}{\mathrm{dt}}$   ...(1)
But momentum P = mV..... (2)
$\therefore \mathrm{F}=\mathrm{k}\frac{\mathrm{d}\left(\mathrm{mV}\right)}{\mathrm{dt}};\mathrm{F}=\mathrm{km}\frac{\mathrm{dv}}{\mathrm{dt}} ..\left(3\right)$
But acceleration
$\mathrm{a}=\frac{\mathrm{dv}}{\mathrm{dt}} ...\left(4\right)$
Eqn (4) is substituting in eqn. (3)
F = Kma
K is proportionality constant
$\text{ If }\mathrm{K}=1\therefore \mathrm{F}=\mathrm{ma}$
b) Yes, If a body is moving along a circular path there should be a force acting on it. The velocity is changing continuously the body will have acceleration, this is normal acceleration or centripetal acceleration.
So, the body is subjected to centripetal force.