Researchers have been able to study the pathway of a secreted protein by tagging it with a fluorescent marker. Using this method, you would observe Fluorescence moving from the ER to which organelle

I would observe fluorescence moving from the ER to the Golgi apparatus.  

Explanation:

Once the protein leaves the endoplasmatic reticulum (ER), it is in a vesicle that fuses with the Golgi apparatus. The Golgi modifies and packs the protein to send it to its destination. In other words, the Golgi complex is the place where proteins go to be functional and be given a place either inside or outside the cell.

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