In case of heat transfer by conduction in a hollow cylinder, __________ mean area is used to calculate the heat transfer rate.

ANS:C - logarithmic

In the case of heat transfer by conduction in a hollow cylinder, the logarithmic mean area is used to calculate the heat transfer rate. The logarithmic mean area takes into account the varying cross-sectional area along the length of the cylinder, providing a more accurate representation of the heat transfer area compared to the geometric or arithmetic mean areas. Therefore, the correct answer is (c) logarithmic.