Answer:
a) 舶 -> 皇 -> 理 -> 黒
舶 HAKU - ship
皇 KOU, OU - emperor
理 RI - reason, logic
黒 KOKU, kuro(i) - black
b) 弦 -> 畜 -> 町 -> 頂
弦 GEN, tsuru - string, bowstring
畜 CHIKU - animal raising, domestic animal
町 CHOU, machi - town
頂 CHOU, itadaki, itada(ku) - top, peak, summit, receive