Answer: D. 4
Explanation:
Hi, to answer this question we have to divide the number given. 
78.4 ÷ 18 = 4.355555556 
The best estimate is 4, because if we round the number to the nearest whole number, since the number after the decimal is less than 5 (.3) the nearest whole number is 4. 
The correct option is D.4. 
Feel free to ask for more if needed or if you did not understand something.