fplot passes a vector of values into the function, so in C, T will be a vector of values. But you are using T as the upper bound on integral(), but integral() can only have a single upper bound.
You need to do something like
C = @(t) arrayfun(@(T) H+(S(1)/S(2))*integral(A,25,T)+(S(3)/S(4))*integral(B,25,T), t);
However, I would recommend that you just be smarter about your integrals. The integral of D1+D2*T+D3*T^2 for T from P to Q is (D1*Q + D2/2*Q.^2 + D3/3*Q.^3) - (D1*P + D2/2*P.^2 + D3/3*P.^3) . That is something that you can easily vectorize, so your C can be calculated without any integral() calls, and so could be vectorized for use with fplot()