Vocabulary in context: in the passage about the power of thought, choose the verb that best completes the clause "a strong force that can be ______ for the welfare and success of all."

Introduction / Context:
The passage concludes by saying that positive thinking is a strong force and that this force can be used for good. The exact phrase with the blank is "positive thinking is a strong force that can be ______ for the welfare and success of all." The verb must describe how a powerful force can be controlled and directed toward beneficial goals.

Given Data / Assumptions:
The force is positive thinking, which has laser like energy seeking a target.
The desired outcome is welfare and success for everyone, indicating constructive use.
Options include released, harnessed, indulgence, liberated and expended.
The tone suggests deliberate use, not mere letting go or wasting of energy.

Concept / Approach:
Harnessed is commonly used to describe the act of controlling and directing a natural force, such as harnessing wind power or harnessing energy, for productive purposes. It is therefore a natural fit when speaking about using the energy of positive thinking for welfare and success. Released and liberated would emphasise letting a force go free without direction. Indulgence is a noun, not a verb, and expended emphasises using something up, not necessarily in a focused way. Thus harnessed is the best verb here.

Step-by-Step Solution:
Step 1: Identify that positive thinking is being compared to a powerful energy that needs to be controlled and guided. Step 2: Recognise that harness is a verb commonly used for controlling horses or for using natural forces productively, and that by extension we harness energy or emotions. Step 3: Test harnessed: a strong force that can be harnessed for the welfare and success of all. This clearly means that the energy is directed toward good purposes. Step 4: Compare released or liberated, which would suggest simply letting the force run free rather than using it constructively. Step 5: Reject indulgence because it is grammatically wrong in this position, and reject expended because it focuses on using something up rather than on purposeful direction.

Verification / Alternative check:
When we read the full sentence with harnessed, the metaphor is strong and coherent: thought is like energy that can be harnessed for good. This parallels common phrases in self help literature about harnessing the power of the mind. If we try released or liberated, the phrase loses the sense of conscious direction and control, which is needed if we want a force to produce specific welfare and success.

Why Other Options Are Wrong:
Released may be appropriate in contexts where energy needs to be let out, but in this passage the focus is on using the energy wisely rather than simply letting it loose. Liberated is similar and usually refers to freeing people rather than using energy. Indulgence is a noun and cannot be used grammatically in the slot that demands a past participle verb. Expended focuses on the consumption of energy without implication of wise guidance; we can expend energy wastefully, which would contradict the idea of welfare for all.

Common Pitfalls:
Students sometimes confuse harnessed and released, thinking that any verb associated with energy must work. It is important to distinguish between letting energy out and directing it. Whenever a passage speaks of energy or power being used for constructive ends, harnessed is a highly likely choice. Paying attention to the phrase for the welfare and success of all guides you to prefer a verb of control and application rather than of mere release.

Final Answer:
The verb that correctly completes the sentence is harnessed, so option harnessed is correct.