Final answer:
Weakened buyer bargaining power is typically indicated by buyers' inability to backward integrate into the business of sellers, implying a dependence that limits their market influence. Option A is correct.
Step-by-step explanation:
The condition that acts to weaken buyer bargaining power among the options provided is A: When buyers are unlikely to integrate backward into the business of sellers. This situation implies a dependence on the sellers, where buyers have minimal leverage over the sourcing of products, potentially increasing their costs and reducing their ability to influence terms and prices. On the other hand, conditions where buyers are well informed, able to switch easily, able to postpone purchases, or are few in number making large purchases, would generally increase buyer bargaining power.