Answer: D. 4
Step-by-step explanation:
In the given picture , we have given a triangle ΔMLN in which
ML= 4x and MN= x+3
∠L=∠N=50°
⇒ MN=ML [∵ Basically by Isosceles triangle property :sides opposite to equal angles of a triangle area equal.]

Subtract x from both sides , we get

Divide bothe sides by x , we get
1= x or x =1
Then, the length of MN
= x+3=1+3=4 units