$\begin{array}{1 1} ab > cd \\ ac >bd \\ ad > bc \\ none\;of\;the\;above \end{array}$

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Answer: (c) ad>bc

Explanation :Sinc a,b,c,d are in HP

$HM of a\;\xi\;c\; is\; b$

$HM of b\;\xi\;d\;is\;c\;$

$GM of a\;\xi\;c\; is\;\sqrt{ac} $

$GM of b\;\xi\;d\; is\;\sqrt{bd} $

Knowing that GM>HM for any two numbers

$\sqrt{ac}>\;b\qquad\sqrt{bd}>c$

$ac\;>\;b^2\;(1)\qquad\;bd\;>\;c^2\;(2)$

$Multiplying (1)\;\xi\;(2)$

$acbd\;>\;b^2 c^2\qquad\;(3)$

$Dividing \;(3)\; by \;bc$

$ad\;>bc\;.$

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