Using expanding foam to adhere tiles is not appropriate due to its properties and behavior when set. Expanding foam is designed for insulation and sealing purposes, expanding as it cures, which can create an uneven substrate and does not provide the necessary rigidity or adhesion for tile. Tile installations require stable, level, and durable materials to ensure proper bonding and longevity.
In contrast, cement and thin-set mortar are specifically formulated for tiling applications, providing strong adhesion that can withstand the weight and stress of the tiles over time. Plywood, while not an adhesive, serves as a substrate in some flooring applications, but it is usually not recommended as a surface for tile adhesion due to its potential for warping and moisture absorption.
Thus, expanding foam is clearly the least suitable choice for adhering tiles, making it the correct answer to the question.