Correct

百思不解

/ bǎi sī bùjiě / 
Strokes

Definition

to remain puzzled after pondering over sth a hundred times (idiom)/to remain perplexed despite much thought

Example

Definition

be still unable to solve a problem after long pondering; remain perplexed despite much thought

1
Tā
他
bù
不
gěi
给
wǒ
我
huíxìn
回信
líng
令
wǒ
我
bǎisībùjiě
百思不解
  
。

I am puzzled by his failure to reply/that he hasn't replied to my letter.

2
Zhè
这
jiàn
件
shì
事
wǒ
我
bǎisībùjiě
百思不解
  
。

The answer still escapes me though I have cudgelled my brains over it.

3
Ràng
让
rén
人
bǎisībùjiě
百思不解

beyond all comprehension