Final answer:
Chaperone proteins are responsible for assisting in the proper folding of proteins within the endoplasmic reticulum by preventing aggregation and ensuring proteins achieve a functional structure.
Step-by-step explanation:
The type of protein that binds to improperly folded or improperly assembled proteins in the endoplasmic reticulum (ER), holding them until proper folding occurs, is C. Chaperone proteins.
Chaperone proteins assist proteins in their folding process by preventing their aggregation during folding, which is crucial to the protein's function. Proteins are indeed specified by their corresponding mRNA but, without proper folding, can become dysfunctional under abnormal conditions such as extreme temperature or pH levels. Chaperones, sometimes referred to as chaperonins, associate with the target protein during the folding process to ensure proper tertiary structure is achieved for functionality.