Answer: [A] < [B]
From the equation, it is clear that 2 moles of A combine with one mole of B.
Let at equilibrium $ a $ moles of B have reacted. This means that $2a$ moles of A have reacted.
$ \therefore $ Equilibrium concentration of A = 2 - 2$ a $
Equilibrium concentration of B = 2 - $ a $
Hence [A] < [B]