Notice the term- 'I disagree'. this disagreement is against the conclusion that the board must be disbanded.
Premise: The threat of having their shipments rejected deters many cheese exporters from
shipping substandard product.
so you see the consultant is disagreeing to the conclusion of the lawmaker by showing the advantage the board elicits due to its presence.

Lets look at options now.

(A) rejecting the lawmaker's argument while proposing that the standards according to which the board
inspects imported cheese should be raised [the consultant does not give any further advice. he just shows the advantage of the existence of a board. Eliminated!]

(B) providing evidence that the lawmaker's argument has significantly overestimated the cost of maintaining
the board [consultant does not attack the cost of maintaining the board at all. Eliminated!]

(C) objecting to the lawmaker's introducing into the discussion factors that are not strictly economic [neither the lawmaker nor the consultant introduce any other external unrelated factor. Eliminated!]

(D) pointing out a benefit of maintaining the board, which'the lawmaker's argument has failed to consider [Correct!this perfectly sits in with our reasoning.]

(E) shifting the discussion from the argument at hand to an attack on the integrity of the cheese inspectors [consultant talks only about the board and not on the integrity of any person. Eliminated!]

Hope this helps!