The back garden of our house contains a lawn,__ very pleasant to sit on in summer.A.which is B.which it is C.it is D.where it iswhy the answer is

此处考查非限制性定语从句，非限制性定语从句是先行词的附加说明，去掉了也不会影响主句的意思，它与主句之间通常用逗号分开，此处由which引导放在句尾
此题个人认为选A
下面有2个类似例子 供参考
非限制性定语从句还能将整个主句作为先行词, 对其进行修饰, 这时从句谓语动词要用第三人称单数，例如：
He seems not to have grasped what I meant, which greatly upsets me. 他似乎没抓住我的意思，这使我心烦。
Liquid water changes to vapor, which is called evaporation. 液态水变为蒸汽，这就叫做蒸发。

It is very pleasant to sit on that lawn in summer. - 变为定语从句：

on which it is very pleasant to sit in summer - 介词后置：

which it is very pleasant to sit on in summer.

逗号隔开,非限定性定语从句
关系代词用which
判断先行词是a lawn （一片草坪）
先行词在定语从句中作sit on 的宾语,不能用where
但此时从句还不完整,
所以要补上it is 作为从句的主语和宾语