The Inverse Document Frequency (IDF) is calculated as:

$IDF = \log\left(\frac{N}{df}\right)$

Where:

• N = total number of documents (sentences) = 2,718
• df = number of documents containing the term = 1,618

Calculation: $IDF = \log\left(\frac{2718}{1618}\right) = \log(1.68)$

Using natural logarithm: $\ln(1.68) \approx 0.52$

Therefore, the IDF is closest to 0.52.