4.Along with the letter was his promise ____ he would visit me this coming Christmas.A.Which B.that C.what D.whetherPlease translate and explain

4.Along with the letter was his promise ____ he would visit me this coming Christmas.
A.Which B.that C.what D.whether
Please translate and explain

B.that
同位语从句~
his promise that he would visit me this coming Christmas这一句话是his promise的同位语~
在信中他还提到了,他答应（保证）在即将来临的圣诞节他回来看我的~