While the analysis of Ludwik Kowalski's given by Herb Schulz is certainly
correct, it misses the problem.
Ludwik's confusion is due to the inconsistant usage of "reaction force"
in many texts and in different situations.
When the block resting on the plate are both at rest, the Normal Force
and the Reaction Force are identical. If the plate and block have a common
acceleration, due to an external force on the plate, then the plate exerts
BOTH a Normal Force AND a Friction Force.
The resultant of these TWO forces CAN be considered the "Reaction Force"
of the plate on the block, and is therefor oblique. The Free-Body Diagram
of the block would then show ONLY TWO forces, the weight of the block
and the oblique reaction force.
It is cleaner, IMHO, to stick with the THREE forces, as Schulz did,
the weight of the block, Normal Force due to the plate, and Friction Force
due to the plate.