Explanation:

- The sentence suggests that machines were originally meant to serve humans.

- Over time, humans have come to heavily rely on machines.

- Option 1: dependent - This implies a reliance on machines, which aligns well with the context that humans have become reliant on machines.

- Option 2: inferior - This suggests a lower status or worth, which doesn’t fit the context of reliance.

- Option 3: subordinate - Although this implies a lower rank, it does not convey the reliance humans have on machines.

- Option 4: helpless - This means unable to act without assistance, which does not capture the idea of dependence.

Option 1: dependent is the most appropriate choice for the blank.