Sealed biscuits become crispy when stored in the refrigerator due to the low humidity environment within the fridge. Refrigerators are designed to keep the air dry, which helps in preserving food by reducing moisture levels. When biscuits are placed in this low-humidity environment, any excess moisture present in them is absorbed by the dry air, effectively removing it from the biscuits. This drying process helps to restore and maintain the crispiness of the biscuits, as moisture is the primary factor that makes them soft and soggy.

Unlike options that suggest high humidity or pressure effects, the actual mechanism involves the refrigerator’s ability to lower humidity, creating a drying effect on the food items stored within. Therefore, the correct answer is [B] Humidity inside the refrigerator reduces and hence excess moisture is absorbed, which keeps the biscuits crispy.