Answer:
correct option is d. $600
Explanation:
given data 
sells product = $20 per unit
selling price fallen = $15 per unit
FIFO inventory = 200 units
purchased = $16 per units
Net realizable value fallen = $13 per unit
to find out
amount of the lower cost of market
solution
we know that here Company record inventory at lower 
so market value or cost of the inventory at  declined time 
and here Market Adjustment is the Difference of the cost and the Market Value
so cost will be here
Cost =  200 × $16 
cost = $3200
and 
Net realizable value will be 
Net realizable value =  200 × $13 
Net realizable value = $2600
so that Market adjustment is the difference of 
Market adjustment  difference = $3200 - $2600 
Market adjustment  difference = $600
so correct option is d. $600