My eldest son, who is in Lagos, studying English.

Analyze the question: The sentence is "My eldest son, who is in Lagos, studying English." (The original is slightly garbled, but it is meant to be "My eldest son, who is in Lagos, is studying English.") We need the option that best explains the information it conveys.

Concept: The word eldest is a superlative used for comparing three or more people. If someone has an "eldest son," they must have more than one son. (If there were only one son, you would not say "eldest.") Also, "who is in Lagos" is a non-defining clause (set off by commas). It gives extra information about the son and does not identify him from among other sons.

Step-by-step check of the options:

• A. Only my son is in Lagos studying English. This implies there is no other person doing so, and it ignores the "eldest" idea. ✗
• B. My only son is in Lagos studying English. "Only son" contradicts "eldest", because an eldest son implies at least one younger brother. ✗
• C. One of my sons is in Lagos studying English. The eldest son is one of several sons, and he is the one in Lagos studying English. ✓
• D. My sons are in Lagos but only one is studying English. This says all the sons are in Lagos, which the sentence does not state. ✗

Answer: The sentence points to one particular son, the eldest, out of several sons.

Tip: In "best explains the information" questions, look at key words such as eldest, only, first, all. "Eldest" signals more than one child, so rule out options that say "only son."