Choose the most appropriate adjective to complete the sentence He has a _______ interest in studying human psychology.

Introduction / Context:
This question tests your knowledge of natural English collocations, that is, words that commonly occur together. The sentence given is He has a blank interest in studying human psychology. Several adjectives are offered, but only one combines with interest in a standard, idiomatic way to express strong, serious involvement. Such collocation questions are important because they show whether you understand not only vocabulary but also typical usage patterns in real language.

Given Data / Assumptions:

The base sentence is He has a blank interest in studying human psychology.

The options are deep, wide, vast, and heavy.

The context suggests a serious, strong, perhaps long lasting interest in an academic subject.

Concept / Approach:
The phrase deep interest is a very common and natural collocation. It expresses that someone is strongly and sincerely interested in a topic, often at an emotional or intellectual level. Wide interest is occasionally used, but usually in the sense that many people are interested, as in the topic generated wide interest among researchers. Vast interest is uncommon and sounds awkward, because vast usually describes size or quantity, not mental attitudes. Heavy interest is not idiomatic in this context; heavy tends to describe physical weight, burdens, or serious responsibility, not interest in a subject. Therefore, deep is the correct adjective to complete the sentence naturally.

Step-by-Step Solution:
Step 1: Identify the noun interest and recall which adjectives are commonly used with it in English. Step 2: Recognise that expressions like a deep interest in science or a deep interest in music are frequently seen in books, articles, and formal writing. Step 3: Compare this with wide interest, which usually describes how widely a topic is followed, not how intensely one person feels about it. Step 4: Consider vast interest and heavy interest, and notice that native usage rarely combines these adjectives with interest in the sense of personal enthusiasm for study. Step 5: Choose deep as the adjective that best expresses a strong, serious, and lasting interest in studying human psychology.

Verification / Alternative check:
Insert each option into the sentence and read it aloud. He has a deep interest in studying human psychology sounds natural and is commonly used in academic introductions or biographies. He has a wide interest in studying human psychology sounds as if his interest covers many areas, but still feels slightly odd; more natural would be a wide range of interests. He has a vast interest in studying human psychology does not match usual patterns, since vast modifies concrete nouns like area or numbers. He has a heavy interest in studying human psychology sounds incorrect, because heavy does not normally modify interest in this way. Checking standard usage in quality reading materials will confirm that deep interest is the established collocation.

Why Other Options Are Wrong:
Option B wide is more often used with words like range, variety, or audience and is not the most natural partner for interest in this sentence. Option C vast typically modifies physical or measurable quantities such as land, knowledge, or resources and rarely appears with interest as a personal feeling. Option D heavy tends to describe weight, work, or responsibility and does not collocate with interest in the sense of enthusiasm for a subject.

Common Pitfalls:
Many learners focus only on the dictionary meanings of adjectives and neglect collocation patterns. Because wide and vast both suggest something large, they may seem suitable for describing the extent of interest. However, natural English relies on fixed patterns such as strong interest, keen interest, and deep interest. Reading extensively and noticing these patterns in context is one of the best ways to master collocations and avoid unnatural combinations in exams and real life communication.

Final Answer:
deep