Answer:
a) 炉 -> 扉 -> 輩 -> 軌
炉 RO - furnace, hearth
扉 HI, tobira - door, title page
輩 HAI - fellow, colleague, companion
軌 KI - wheel track, rut, rail, orbit
b) 炎 -> 畑 -> 町 -> 頂
炎 EN, honou - flame
畑 hatake - cultivated field
町 CHOU, machi - town
頂 CHOU, itadaki, itada(ku) - top, peak, summit, receive