 Solution

Assume $L$ is linear, annihilates all polynomials of degree at most $k$, is defined on the truncated-power kernels and permits interchange with the remainder integral. For $k\ge 1$, a convenient sufficient condition is that $L$ be a bounded linear functional on $C[a,b]$; more general approximation functionals need the corresponding continuity/interchange property on their domain. Taylor's integral remainder givesApplying $L$, the polynomial terms vanish, and the Peano kernel theorem givesHere $t_{+}=\max (t,0)$. This is the printed raw-kernel convention; the normalized Peano kernel includes $1/k!$ inside its definition instead, and then has no factorial outside the integral. The interchange is justified by boundedness applied to the limit of Riemann sums of the continuous kernel-valued integrand. For a kernel with $k=0$, its discontinuity requires a function space/interchange assumption accommodating the step functions as well.

For the specified Simpson's rule error, direct evaluation on $1,x,x^2,x^3$ gives zero, so $k=3$. For $0\le \theta \le 2$,Simplifying separately on the two halves givesBoth pieces are strictly negative in the open interval; they agree at the midpoint. Their integral is $-1/15$, as direct polynomial integration shows. ThusThis is optimal, not merely an upper bound. For $f(x)=x^4/24$, the fourth derivative is identically one and $L(f)=-1/90$, so the minimum constant is $\rho =1/90$.