The boy had been warned not to … the money given to him.

Analyze the question: The sentence is "The boy had been warned not to __ the money given to him." We need a verb that fits after "not to" (the infinitive form), and it must mean "to no longer have something" or "to misplace it."

Explain the concept: This question tests commonly confused words:

• lose (verb, /luːz/) means to misplace something or fail to keep it. Example: "Don't lose your pen."
• loose (adjective, /luːs/) means not tight or not fastened. Example: "a loose tooth."
• lost is the past tense or past participle of lose. Example: "He lost his bag."
• loosen (verb) means to make something less tight. Example: "loosen your tie."

After "not to," we need the base form of a verb. So "lost" is out, since it is a past form.

Step-by-step solution:

1. "Not to ___" requires a base-form verb, so C (lost) is wrong.
2. "Loose" is an adjective (or, rarely, a verb meaning to release), and it doesn't make sense with money here, so B is wrong.
3. "Loosen" means to make something less tight, which doesn't fit with money, so A is wrong.
4. "Lose" means to misplace or be deprived of something, which fits perfectly: "not to lose the money."

Answer: The correct option is D. lose.

Exam tip: Remember that lose has just one "o" and means "to misplace," while loose has two "o"s, like "too much room," meaning "not tight." Also check the grammar: after "to," always use the base verb form.