Read the following sentence and identify the part that contains an error in standard English usage: "I am senior than him by two years."

Introduction / Context:
This error spotting question focuses on correct comparative structures with adjectives like "senior", "junior", "inferior", and "superior". These adjectives behave differently from regular comparative forms because they do not use "than" and instead take the preposition "to". Many learners incorrectly say "senior than" or "junior than" by analogy with "older than". The exam tests whether you know this special rule.

Given Data / Assumptions:
The sentence is divided into parts:
- Part (a): "I am senior"
- Part (b): "than him"
- Part (c): "by two years"
- Part (d): "No error"
We must locate the portion containing the grammatical error, if any.

Concept / Approach:
Certain adjectives that express rank or status traditionally take "to", not "than", when making comparisons. These include "senior", "junior", "superior", "inferior", "prior", and a few others. Therefore, the correct expression is "I am senior to him by two years", not "senior than him". The rest of the sentence, including the phrase "by two years", is acceptable and common in English when describing the age or service difference between two people.

Step-by-Step Solution:
Step 1: Identify the adjective and comparison pattern: "senior than him". Step 2: Recall the special rule that "senior" takes "to", not "than". Step 3: Replace "than him" with "to him" to form "I am senior to him by two years". Step 4: Check whether any other part of the sentence is incorrect. "I am senior" and "by two years" are both fine. Step 5: Conclude that the error is in part (b), which reads "than him".

Verification / Alternative check:
Compare with similar examples: "She is junior to me by three years", "This post is superior to that one in rank". None of these take "than". On the other hand, adjectives like "older" or "taller" do take "than": "I am older than him by two years". The mixture of these rules often causes confusion, but for adjectives such as "senior", "to" is required in formal usage.

Why Other Options Are Wrong:
- Part (a) "I am senior": forms the beginning of a correct comparative expression and is not wrong by itself.
- Part (c) "by two years": correctly expresses the margin of difference and is standard in English.
- "No error": is incorrect because we have clearly identified an error in part (b).

Common Pitfalls:
Many examinees mechanically apply the "adjective plus than" pattern learned for simple comparatives without remembering that there are exceptions. A helpful memory trick is to group these special adjectives together: senior to, junior to, superior to, inferior to, prior to. If you see any of these followed by "than", it should immediately signal a likely error in error spotting questions.

Final Answer:
The incorrect portion is than him, that is, part (b). The corrected sentence is "I am senior to him by two years."