Among the provided options, which expression indicates that F is the mother of K?
Given the following codes:

* $\text{B5D}$ means B is the father of D
* $\text{B9D}$ means B is the sister of D
* $\text{B4D}$ means B is the brother of D
* $\text{B3D}$ means B is the wife of D

Let's decode option B ($\text{F3M4N9K}$):
1. $\text{F3M}\to$ F is the wife of M (So, F is female and M is male).
2. $\text{M4N}\to$ M is the brother of N.
3. $\text{N9K}\to$ N is the sister of K.

Since M, N, and K are siblings (M is brother of N, N is sister of K), and F is the wife of M, F would be the sister-in-law of K, not the mother.

Let's decode option C ($\text{F3M5K}$):

1. $\text{F3M}\to$ F is the wife of M (F is female, M is male).
2. $\text{M5K}\to$ M is the father of K.

Since M is the father of K and F is M's wife, F must be the mother of K. This perfectly matches the requirement.