A block A placed on an inclined surface just starts to slide when...

The angle of repose $\theta$ is the minimum angle of inclination of a plane at which a block placed on it just begins to slide.

The condition for the block to just start sliding is $mg \sin \theta = \mu_s mg \cos \theta$, which simplifies to $\tan \theta = \mu_s$, where $\mu_s$ is the coefficient of static friction between the block and the inclined surface.

The angle of repose depends only on the nature of the surfaces in contact (coefficient of friction) and is completely independent of the mass of the block.

Since blocks $A$ and $B$ have different angles of repose (30^{}^{{}^{\circ}} and 40^{}^{{}^{\circ}} respectively), their coefficients of friction with the inclined surface are different. This difference provides no information about their masses.

Therefore, the mass of block $A$ can be less than, greater than, or equal to the mass of block $B$. All three cases are possible.