Answer: The fourth option. 

Explanation:
BA stands for the segment between B and A, so this is not the correct option. 
B (with a double arrow on top) refers to a line, but with only one point we could draw infinite lines, so this is not the correct option) 
mB (with a double arrow on top) does not mean anything. 
AB (with a double arrow on top) refers to a line that phases through the points A and B, so this is the other way of naming the line m.