Answer:
Option (B) is correct.
Step-by-step explanation:
For a 20 workday month, 
cost of gas and productivity = $4 per day 
cost of commuting = cost of gas and productivity × 20 workday month
 = $4 × 20 
 = $80. 
The total rent he is paying currently is $600 per month that does not include the commuting cost. 
Hence, the individual must willing to pay a total of: 
= Total rent + Cost of commuting 
= $600 + $80 
= $680 for an apartment downtown. 
 
Thus, the total amount to be paid willingly is $680.