Identify from the following the sentence with correct use of preposition.
a. She has been teaching in this college from 2001.
b. She has been teaching in this college since 2001.
c. She has been teaching in this college before 2001.
d. She has been teaching in this college for 2001.

The correct answer is Option D (b only).

Key Points:
• Concept: Use of 'Since' vs 'For' in Perfect Continuous Tenses.
• Since is used to denote a specific point in past time (e.g., since 2001, since morning).
• For is used to denote a duration/period of time (e.g., for 5 years).
• Explanation: In the present perfect continuous sentence 'She has been teaching...', the year '2001' represents a precise starting point in time. Therefore, 'since 2001' in sentence 'b' is the only grammatically correct structure.
• Other Sentences: 'from' is inappropriate with present perfect continuous; 'for' requires a duration, not a point in time.
• Official Key: Option 4 ('b only') is the official answer.
• Key Takeaway: Remember: Use 'since' for a specific point in time and 'for' for a period of time.