dehydration of alcohol order
Consider these two factors -
1 Dehydration of alcohols follows SN1 mechanism through carbocation and more stable carbocation (30 ,20, 1 0) formed is more stable .
2 More substituted alkene is also most stable product (Zaitsev's rule)
Now 30 alcohol will be most reactive because it is 30 and forms most substituted alkene and 10 is least reactive. In 10 pick up the alcohol which gives you least substituted alkene.
Based on these factors predict the right answer.