To solve this problem, we need to analyze the forces acting on the block.$\\$Consider the block of mass $m$ on the inclined plane with inclination $\theta. \\$The wedge is accelerating to the right with acceleration $a. \\$For the block to remain stationary relative to the wedge, the net force parallel to the incline must be zero.$\\$Let's analyze the forces:$\\$• The gravitational force component along the incline is $mg \sin \theta. \\$• The normal force $N$ acts perpendicular to the incline.$\\$• The pseudo force due to the acceleration of the wedge acts horizontally to the left with magnitude $ma. \\$Resolve the pseudo force along the incline:$\\$• The component of the pseudo force along the incline is $ma \cos \theta. \\$For the block to remain stationary, the net force along the incline must be zero:$\\ mg \sin \theta = ma \cos \theta \\$Rearrange to find the relation between $a$ and $\theta: \\ a = g \tan \theta \\$Therefore, the correct option is Option 4: $a = g \tan \theta.$