b
The candle floats on the water with half its length above and below water level. Let its length be \(10\, cm\). with \(5\, cm\). below the surface and \(5\, cm\). above it. If its length is reduced to \(8\, cm\). It will have \(4\, cm\). above water surface. So we see tip going down by \(1\, cm\). So rate of fall of tip \(= 1\, cm/hour\).