He came in as quietly as he could _____ not to wake the children.

because of

so as

although

whether

He came in as quietly as he could so as not to wake the children.

وہ جتنی خاموشی سے کر سکتا تھا اندر آیا تاکہ بچوں کو جگا نہ دے۔

"so as" is best to keep the meaning of the original sentence

"so as" means with the result or purpose.