Error spotting in an English grammar sentence: Though he is a gifted comedian, he prefers spend his spare time watching horror movies. Identify the part of the sentence which contains the grammatical error or decide that there is no error.

Introduction / Context:
This question tests basic English grammar through an error spotting sentence. The sentence is divided into four labelled parts, and the learner is asked to find out which specific segment contains a grammatical mistake. Such questions are very common in competitive examinations because they check the candidate's understanding of verb patterns and infinitive usage in real life like sentences about hobbies and preferences.

Given Data / Assumptions:
- The full sentence reads: Though he is a gifted comedian, he prefers spend his spare time watching horror movies.
- Part (a): Though he is a gifted comedian
- Part (b): he prefers spend his spare time
- Part (c): watching horror movies
- Part (d): No error
We assume standard modern English usage appropriate for competitive exams and formal writing.

Concept / Approach:
The key grammatical area here is the verb pattern that follows the verb prefer. In English, prefer is normally followed either by a noun or by the structure prefer to plus base form of the verb. For example, we say prefer tea, prefer coffee, or prefer to watch movies. When prefer is followed by another verb, the infinitive marker to is required before the base form. Therefore prefers spend is not acceptable; it must become prefers to spend. The other clauses in the sentence follow correct patterns for a complex sentence introduced by though.

Step-by-Step Solution:
Step 1: Examine part (a): Though he is a gifted comedian. This is a correct concessive clause using though followed by subject, verb, and complement. Step 2: Examine part (b): he prefers spend his spare time. Here the verb prefers is immediately followed by the bare infinitive spend without the marker to. Step 3: Recall the correct pattern: prefer to do something. So the correct phrase should be he prefers to spend his spare time. Step 4: Examine part (c): watching horror movies. This is a correct participial phrase that describes how he spends his spare time. Step 5: Since parts (a) and (c) are correct and only part (b) has a verb pattern error, the error is confined to part (b) only.

Verification / Alternative check:
We can rewrite the whole sentence in a corrected form: Though he is a gifted comedian, he prefers to spend his spare time watching horror movies. This corrected version reads smoothly and follows the standard rule for prefer. No other changes are required for sense or grammar. The meaning remains that in spite of being a comedian, his free time activity is watching horror movies. This cross check supports the conclusion that the only necessary correction is in part (b).

Why Other Options Are Wrong:
- Error in part (a) only: Part (a) is grammatically correct; the structure though he is a gifted comedian is standard.
- Error in part (c) only: Watching horror movies is a correct expression functioning as an object complement; there is no error here.
- No error in the sentence: This is incorrect because prefers spend is not an accepted standard pattern.
- Error in more than one part: Only one part needs correction, so this option is also incorrect.

Common Pitfalls:
Many learners read quickly and focus only on vocabulary, overlooking small structural errors in verb patterns. Another common mistake is to think that prefer can be followed directly by a bare verb because similar verbs like like or love sometimes allow a gerund. It is also easy to assume that the long phrase about spare time may contain an article or preposition error, but the real issue is the missing to before spend. Candidates should consciously check verb plus infinitive patterns in error spotting questions.

Final Answer:
Correct option: Error in part (b) only.