Explanation:

Main memory, specifically Random Access Memory (RAM), is where the CPU stores data and instructions that it needs to access quickly. The CPU constantly interacts with main memory to fetch instructions, read data, and write results. This close cooperation is essential for the computer's operation. While RAM is main memory (making option b redundant in this context as a 'conjunction' partner), the CPU is the primary component that works in conjunction with it to execute tasks. Special function cards (a) are peripheral components, and Intel (d) is a manufacturer, not a component that works in conjunction with main memory in the same operational sense as the CPU. Therefore, the CPU is the correct answer as it directly and continuously collaborates with main memory.