Step 1: Understanding the Problem:
In 1983, the American company Eli Lilly successfully prepared the first recombinant human insulin (Humulin) by independently synthesizing its two short polypeptide chains.
Step 2: Approach and Formula:
Sequence the experimental steps taken by Eli Lilly according to the standard workflow of Recombinant DNA Technology.
Step 3: Detailed Explanation:
The sequential procedure executed by Eli Lilly was:
1. First, they artificially prepared two DNA sequences corresponding strictly to the amino acid sequences of chain A and chain B of human insulin (Step a).
2. Next, they introduced these DNA sequences into plasmids and transformed them into {E. coli} bacterial cells (Step c).
3. Inside the bacterial host, the recombinant genes were expressed, resulting in the production of chain A and chain B separately in different bacterial cultures (Step b).
4. Finally, they extracted the separated chains from the bacteria and covalently combined them by creating disulfide bonds to form fully mature, functional human insulin (Step d).
Therefore, the chronological order is (a) $\rightarrow$ (c) $\rightarrow$ (b) $\rightarrow$ (d).
Step 4: Final Answer:
The correct sequence is (2).