When measuring readability, the average number of words per sentence is significant for which index?

The Gunning Fog Index is significant for measuring readability because it takes into account the average number of words per sentence as a key component of its calculation. The Fog Index is designed to assess the readability of English writing by making the reader aware of the complexity of the text. The formula incorporates the average length of sentences and the percentage of words that are three syllables or more. A higher Fog Index score indicates that the text is more difficult to read and may require a higher level of education to understand.

This focus on sentence length captures the idea that longer sentences can be more complex and harder to follow, thus influencing the overall readability. While other indexes also consider sentence length and complexity, the Gunning Fog Index explicitly emphasizes the number of words per sentence in its formula, which is why it is the correct answer in this context.