How do we measure cloud cover?

✅Explanation:
Okta is the unit used to measure cloud cover, where the sky is divided into 8 parts (oktas), and each part represents the extent of cloud cover. The percentage of the sky covered by clouds can also be expressed, with 0% meaning clear skies and 100% meaning completely overcast.
Other Options Explanation
-a) Decibel: Decibels are used to measure sound intensity.
-c) Richter Scale: The Richter scale is used to measure the magnitude of earthquakes.
-d) Beaufort Scale: The Beaufort scale is used to measure wind speed.