Final answer:
In multiplex PCR, a significant decrease in template amount would generally lead to poor or failed amplification, allelic dropout, and possibly longer targets amplifying poorly, but not increased yield heterozygote imbalance.
Step-by-step explanation:
If the amount of template in a multiplex PCR fell significantly below the optimal amount, the least expected outcome would be d. increased yield heterozygote imbalance. The more common results would likely include longer targets amplifying poorly or failing to amplify (b), allelic dropout (c), and potentially competition among primer sets affecting amplification efficiency. The low amount of template DNA would generally lead to insufficient target DNA for the PCR to effectively amplify, and thus, decreased overall yield rather than an increased yield heterozygote imbalance.