The most likely to take place when a solid is placed in a liquid at room temperature ( i.e. 20°C ) is ; ( A ) A chemical reaction
Although your question lacks the required diagram a general answer is provided within the scope of your question
When a solid at 20°C is placed inside a liquid also at 20°C in the laboratory the emergence of gas bubbles shows that there has been a chemical change between the solid and liquid reactant ( i.e. a chemical reaction has taken place ).
Hence we can conclude that what is most likely to have taken place in this situation is a Chemical reaction.
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