This definitely kicks the difficulty of comparison questions up a notch! Great question for people who have the basics of comparisons down and want to try something a bit harder.
Unlike the previous year’s bidding, the contract was awarded not simply to the firm offering to complete the work on time for the least cost; the thoroughness of the design submission was also factored into the decision
Unlike the previous year’s bidding, the contract this year was awarded not simply to the firm offering to complete the work on time for the least cost; - This is a comparison error. It compares the previous year's bidding to the contract.
This year, unlike last year, the contract was awarded not simply to the firm offering to complete the work on time for the least cost; - This is the correct answer. Correctly compares this year to last year.
Unlike the previous year’s bidding, this year the contract was awarded not simply to the firm offering to complete the work on time for the least cost; - Another comparison error. The previous year's bidding compared to this year.
Unlike the previous year’s bidding, the bidding for the contract this year was awarded not simply to the firm offering to complete the work on time for the least cost, instead - The comparison is corrected here but the comma followed by the word instead creates a run on sentence.
Unlike the previous year’s bidding, the contract’s bidding this year were awarded not simply to the firm offering to complete the work on time for the least cost; - Incorrectly compares the previous year's bidding to the contract's bidding.
Take Away - Be wary of comparison questions where ownership can be used to muddle what is being compared. If you quickly read "the previous year's bidding" you could easily incorrectly think that it's the year that is being compared to something not the bidding of that year.
Also, a good takeaway is to know what words can be used to link two independent clauses. I originally incorrectly assumed instead was one of these and picked D.