Complete the sentence by choosing the correct preposition: "I have decided _____ my course of action."

Introduction / Context:
This sentence-completion question tests your knowledge of fixed verb-preposition combinations, also known as collocations. The verb "decide" is commonly followed by prepositions in different patterns, and choosing the correct one is important for natural and correct English usage. Here, the speaker is talking about having made a final choice about a course of action.

Given Data / Assumptions:
- Sentence: "I have decided _____ my course of action."
- Options: upon, with, for, of.
- We must pick the preposition that fits standard English collocation and conveys the sense of having made a decision.

Concept / Approach:
When "decide" is followed by a noun phrase (not directly by a verb), English often uses "on" or "upon". For example: "I have decided on a plan" or "They decided upon a strategy." "Decide with", "decide for", and "decide of" are either incorrect or used in different structures. In the given sentence, "upon" fits naturally and is traditionally used in formal contexts.

Step-by-Step Solution:
Step 1: Focus on the pattern: "decided _____ my course of action". We need a preposition that connects "decided" to the noun phrase "my course of action".
Step 2: Test "upon": "I have decided upon my course of action." This is a standard, formal-sounding sentence.
Step 3: Test "with": "I have decided with my course of action" sounds wrong; "with" usually connects people, as in "decided with my team".
Step 4: Test "for": "decided for my course of action" is not idiomatic here; "decided for" is used in structures like "decided for the children" meaning on their behalf.
Step 5: "Of" does not work in this structure either ("decided of" is incorrect).

Verification / Alternative check:
Compare similar sentences: "She has decided on a career in medicine" and "They decided upon a compromise." In each case, "on/upon" links the act of deciding to the object of that decision. Replacing "upon" in the given sentence with "on" would also be correct, but "upon" is one of the traditional choices in formal exam-style English, and among the provided options it is the only suitable one.

Why Other Options Are Wrong:
- With: typically used for companionship or association ("with my friend"), not for connecting "decide" to its object.

- For: used in contexts like "decide for someone", meaning to make a decision on another person's behalf, not with a course of action as the object.
- Of: does not collocate with "decide" in this structure.

Common Pitfalls:
Students sometimes overgeneralise "for" after many verbs, but "decide" has more specific patterns. To avoid errors, remember two main patterns: "decide to do something" (verb after decide) and "decide on/upon something" (noun after decide). Once you fix these patterns in your mind, choosing the right preposition in exam questions becomes much easier.

Final Answer:
The correct preposition is "upon": "I have decided upon my course of action."