Correct Answer - C
When object is at distance 15 cm, magnification is `2lamda` when object is at distance 20 cm, magnification is `lamda`
`m_1=-lamda,u_1=-20cm`
`m=-(v)/(u)implies-lamda=-(v_1)/(-20)`
`v_1=-20lamda`
`m_2=-2lamda,u_2=-15cm`
`m=-(v)/(u)implies-2lamda=-(v_2)/(-15)`
`v_2=-30lamda`
`(1)/(f)=(1)/(v_2)+(1)/(u_2)=(1)/(v_1)+(1)/(u_1)`
`(1)/(-30lamda)+(1)/(-15)=(1)/(-20lamda)+(1)/(-20)`
`(1)/(20lamda)-(1)/(30lamda)=-(1)/(20)+(1)/(15)`
`(1)/(60lamda)=(-3+4)/(60)implieslamda=1`
`(1)/(f)=(1)/(v_2)+(1)/(u_2)=(1)/(-30lamda)+(1)/(-15)`
`=(1)/(-30)+(1)/(-15)=(-(1+2))/(30)`
`f=-10cm`