The correct option is C 72th
56,5515,5425,⋯
=2805,2765,2725,...
Hence, the given sequence is in AP with common difference d=−45
To find the first negative term, Tn<0
⇒a+(n−1)d<0
⇒56+(n−1)(−45)<0
⇒280−4n+4<0
⇒4n>284
⇒n>71
Hence, first negative term is 72th.