Answer: AB + BC' + BD'
Output of NAND gate which has input As D and C is (DC)'
Output of NAND gate which has input As A and B is (AB)'
Output of NAND gate which has input As (DC)' and B is ((DC)' . B)'
Output of NAND gate which has input As ((DC)' . B)' and (AB)' is (((DC)' . B)' . (AB)')'
If we solve it (((DC)' . B)' . (AB)')' then
(DC)' . B + AB
(D' + C' )B + AB
BD' + BC' +AB