In a cloze assessment, which area of reading proficiency does the teacher primarily assess by omitting every fifth word from a passage?

The correct choice focuses on the assessment of knowledge of syntax and vocabulary in a cloze assessment where every fifth word is omitted from a passage. This type of assessment requires students to utilize their understanding of sentence structure (syntax) and word meanings (vocabulary) to fill in the blanks appropriately.

By removing specific words, the test evaluates the learners' ability to comprehend how words fit in context and how they function grammatically within sentences. Students need to recognize not just the meaning of the omitted words but also their grammatical role in the sentence structure, which directly ties into their knowledge of syntax and vocabulary.

Other areas such as background knowledge may influence responses but are not the primary focus of this specific assessment format. Similarly, while fluency in reading aloud and the ability to summarize texts are important skills, they are not directly evaluated in this scenario where the emphasis is placed specifically on vocabulary and syntax awareness.