Among the provided options, which expression indicates that F is the mother of K?

Given the following codes : B5D means B is the father of D, B9D means B is the sister of D, B4D mean B is the brother of D, and B3D means B is the wife of D.

Decoding digits:

So, Checking each option one by one

Option 1) F3M4N9K → F is the wife of M who is the brother of N who is the sister of K.

Here, F is sister-in-law of K.

Option 2) F9M4N3K → F is the sister of M who is the brother of N who is the wife of K.

Here, F is sister-in-law of K.

Option 3) F5K3M → F is the father of K who is the wife of M.

Here, F is father of K.

Option 4) F3M5K → F is the wife of M who is the father of K.

Here, F is the mother of K.

Hence, the correct answer is "Option 4".