ClearAll["`*"]
TC = Rationalize[
1/T {Ch*D1 ((\[Lambda]*(c*(s - 1) - td)*(T - td)^2)/
2 + (a*(T + c*(1 - s))*(T - td)^b + 1)/(b +
1) - (2*a*(T - td)^b + 2)/((b + 1)*(b + 2)) +
T^2/2 - ((T^3) \[Lambda])/3 +
T^2*\[Lambda]*td - (3 T*\[Lambda]*td^2)/
2 + (\[Lambda]*td^3)/6) + k} /. {D1 -> 4000, c -> 10,
Ch -> 2, s -> 0.1, k -> 200, a -> 0.2, b -> 1.5,
td -> 20/365, \[Lambda] -> 0.2}, 0][[1]] // FullSimplify
(*(200 + 8000 (2334134/5835255 - 4/35 (2 + 2/5 (-(4/73) + T)^(3/2)) - (
11203068469727 (-(4/73) + T)^2)/12372526449169 - (24 T)/26645 + (
373 T^2)/730 - T^3/15 + 2/25 (-(4/73) + T)^(3/2) (9 + T)))/T*)
func = D[TC, T] // FullSimplify
(*-(2850630093888538400/903194430789337) +
960 Sqrt[-(4/73) + T] + (-(156639186421396108130600/
101075585555203914333) + 151040/511 Sqrt[-(4/73) + T])/T^2 + (
18880 Sqrt[-(4/73) + T])/(7 T) - (3200 T)/3*)
Solve[func == 0, T] // N
(*{{T -> -3.03984 - 0.158213 I}, {T -> -3.03984 + 0.158213 I}, {T ->
0.582782 - 0.712647 I}, {T -> 0.582782 + 0.712647 I}}*)