Ah, the joys of multiple-choice questions! Let's unravel this mystery. The sentence structure is key here: "We accomplish this by continually improving our systems of (4) _____."

In grammar, when you're talking about the function or purpose of a system, you are likely to refer to the noun form that describes the activity or process. In this context, "systems of operation" makes sense because "operation" is the noun that indicates the action or process that the systems are designed to perform or improve.

The other options are forms of the verb "operate," which don't fit well grammatically in this context. "To operate" would indicate an incomplete infinitive verb phrase, "operates" is a verb that would require a subject, and "operated" is a past participle that doesn't logically follow "systems of."

So, choice D, "operation," is the correct answer because it's the noun that completes the phrase logically and grammatically. It's like always choosing cake over broccoli – it's an easy choice when you know what you're looking for!

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