Solution
= Solution
If $K_1\leq K_2$, then pointwise
$$
(S_T-K_1)^+\geq(S_T-K_2)^+.
$$
The positive pricing formula under the <T-forward measure> gives
$$
C_t^{T,K}=B_t^T\mathbb E_{Q^T}[(S_T-K)^+\mid\mathcal F_t].
$$
Therefore $C_t^{T,K_1}\geq C_t^{T,K_2}$, so the call price is non-increasing in strike.