Cell biologists studying the mobility of plasma membrane proteins fused mouse and human cells together, creating a heterokaryon. They then examined the localization of mouse and human proteins over time. Initially the mouse and human proteins were restricted to separate halves of the heterokaryon. Over time the mouse and human proteins mixed, with each protein eventually being present over the entire cell surface, consistent with proteins being able to move within the plane of the membrane. What techniques could have been used to examine the locations of the mouse and human membrane proteins

Fluorescence microscopy

Explanation:

A protein is substance that is the essential constituent of living cells because it forms part of its structure. They are also found in plasma membrane, where they assist substances to cross the membrane.  Proteins can be:

Some integral proteins can act as cellular receptors. Other proteins are responsible for cell adhesion (binding of a cell to another cell or to a surface). On the outside of the cell membranes, attached to other proteins, are the carbohydrate chains that act as labels identifying the type of cell.

A heterokaryon is a multinucleate cell and in this experiment the scientists fuse a human cell and a mouse cell, each of them will have their own proteins. At first, the human and mouse proteins where found in separated halves of this heterokaryon. But after a while, those proteins where mixed and could no longer be identified.

A fluorescent chemical called a fluorophore is able to be absorb light of specific wavelengths and then emit light of longer wavelengths. The proteins can be modified and marked with different colored fluorophores to detect them, even if they are mixed in the heterokaryon. Then, they can be seen with a Fluorescence microscopy, to identify them through different colors.  

the answer will be a) proteins i believe