Questions & Answers about Je pose ma brosse à dents sur le lavabo.
Both can work, but they are not exactly the same.
- mettre = a very general to put
- poser = to place / to set down, often onto a surface
So poser sounds a bit more specific here because the toothbrush is being placed on the sink.
Compare:
- Je mets ma brosse à dents sur le lavabo. = I put my toothbrush on the sink.
- Je pose ma brosse à dents sur le lavabo. = I set my toothbrush down on the sink.
Both are natural, but poser matches the idea of placing something onto a surface especially well.