In this case, the market for good X consists of two consumers whose individual own price demand curves are given as X1=9-3px and X2=6-px+X1/3 where X1 and X2 are the quantity demanded by the first and the second consumers respectively and px is the market price of good X.

Now, when the market price of good X is $2 or px=2, the quantity demanded by the first counsumer would be:-

X1=9-3px

X1=9-3*(2)

X1=9-6

X1=3

When the market price of good X is $2 the quantity demanded of good X by the first consumer would be 3 units.

The total market demand function of good X in the market of X would be=X1+X2 and when the px is 2 the X1 has been calculated as 3.

Therefore, when the px is 2 and X1 is 3, the quantity demanded of good X by the second consumer would be:-

X2=6-px+X1/3

X2=6-2+3/3

X2=6-2+1

X2=5

When the market price of good X is $2 the quantity demanded of good X by the second consumer is 5 units.

Therefore, the total market demand for good X when the market price of good X is $2 or px=2 would be=(3 units+5 units)=8 units. Thus, the answer, in this case, would be option d. given in the answer choices or options or 8.