If $*abcd*=a\times d+b\times c$, then $*2543*=$~\uline{~~~~~~~~~~}~. ($2004$ Math League.com contest problem, $8$\textsuperscript{th} Grade, Question \#$33$) 
Overseas Competition->Knowledge Point->Calculation Modules->Operations with New Definition->Operating Directly
$*2543*=2\times3+5\times4=6+20=26$.  So the answer is $\rm C$. 