Scientific articles ___ often ___ in this paper. ?

The correct completion of the sentence is 'Scientific articles are often published in this paper.' This choice is grammatically accurate and semantically clear.

Option A, 'to be / published', is incorrect because 'to be' does not form a proper verb phrase that fits with 'scientific articles' in this context. The phrase lacks a suitable tense and fails to express an action occurring.

Option B, 'are / being published', suggests a continuous action but incorrectly implies that the articles are currently undergoing the process of publication rather than being published regularly. This does not align well with the intended meaning of stating that articles are frequently published.

Option D, 'have / published', also misses the mark because it suggests a completed action without clarity on the subject, making it sound as if the articles have already been published rather than that this is an ongoing occurrence.

In contrast, option C, 'are / published', effectively communicates a general truth about the frequency of publication. 'Are' is the correct present tense form that matches the subject 'scientific articles', and 'published' serves as the main verb in the passive voice, indicating that articles are regularly made available through the paper. Thus, it conveys the intended meaning clearly and accurately.