Fill in the blank with the most appropriate word. I never go shopping on weekends as on those days the malls are ________ of people.

Introduction / Context:
This question tests your understanding of common collocations in English, particularly how certain adjectives combine with prepositions to form natural expressions. The sentence describes the situation in malls on weekends and you must choose the word that best fits the phrase of people.

Given Data / Assumptions:
Sentence: I never go shopping on weekends as on those days the malls are ________ of people.
Options: full, busy, crowded, packed.
We assume a neutral, conversational tone in standard English.
The preposition of is fixed after the blank in the sentence.

Concept / Approach:
Different adjectives prefer different prepositions. We say full of people, crowded with people, and packed with people in natural English. Because the sentence already provides of after the blank, the adjective that best fits the pattern is full. Busy describes activity but does not fit comfortably with of people in this context.

Step-by-Step Solution:
Step 1: Examine the structure: are ________ of people. Ask which adjective normally takes of with people. Step 2: Full of people is a very common expression meaning that there are many people inside a place. Step 3: Crowded usually takes with, as in crowded with people, not crowded of people. Step 4: Packed also normally takes with, as in packed with people, not packed of people. Step 5: Busy of people is not idiomatic; we would say busy with people or simply very busy. Step 6: Therefore, the word that correctly completes the phrase is full.

Verification / Alternative check:
Try reading the full sentence with each option: the malls are full of people sounds natural; the malls are crowded of people does not; the malls are packed of people also sounds wrong; the malls are busy of people is clearly incorrect. This quick substitution test confirms that full is the best and grammatically correct choice.

Why Other Options Are Wrong:
Busy focuses on the level of activity rather than the number of people and does not naturally take the preposition of in this collocation.
Crowded expresses the idea of too many people, but it combines with with people, not of people, in standard English.
Packed is similar to crowded and also prefers with, as in packed with people, so packed of people is ungrammatical.

Common Pitfalls:
Learners may focus only on meaning and think that crowded or packed are stronger expressions for a full mall, ignoring the preposition of that follows. Examinations often test this kind of preposition sensitive collocation, so always check what preposition the adjective normally takes.

Final Answer:
The correct completion is full, giving the sentence: the malls are full of people.