After studying biology all afternoon, Marcus is having difficulty remembering details of the organic chemistry material he memorized that morning. Marcus' difficulty BEST illustrates interference.

Explanation:

Interference theory is a type of theory that explains the human memory. On the basis of certain characteristics, memory is classified as long term memory and short term memory. When some memories are deleted or forgotten from long term memory then such type of memory can not be retrieved by short term memory due to interfering with each other. Thus interference can be grouped as proactive and retroactive interference or learning.  

In the case of retroactive interference, when some events or learning are going on then it is impossible to learn or retrieve some other things. So Marcus' difficulty can be explained by retroactive interference.

the answer will be a) proteins i believe