see how our problem of size n is reducing...like
n -> logn ->loglogn -> loglogn ..................-> 1
it must b need 2 give there that T(1) is constant
now see defn of log*n "In computer science, the iterated logarithm of n, written log* n (usually read "log star"), is the number of times the logarithm function must be iteratively applied before the result is less than or equal to 1. "
so there r log*n such iterations nd cost of each is c hence it is clog*n