Mohan says, "This girl is the wife of the grandson of my mother." How is Mohan related to that girl?

Introduction / Context:
This problem is a classic example of generation based blood relation reasoning. The phrase "wife of the grandson of my mother" needs to be converted into a simple family relationship. The ultimate goal is to find how Mohan stands with respect to the girl he is talking about.

Given Data / Assumptions:

• Mohan is the speaker.
• He refers to "my mother", who is one generation above Mohan.
• The "grandson of my mother" is a male grandchild of Mohan mother.
• The girl in question is "the wife of that grandson".
• We assume the normal situation that the grandson referred to is Mohan own son in typical exam style unless the wording forces a different interpretation.

Concept / Approach:
First we need to locate the grandson. Mohan mother has children, including Mohan himself and possibly Mohan siblings. The sons of those children are her grandsons. The most straightforward and exam friendly interpretation is that Mohan is one of her children and that her grandson here is Mohan own son. Once that is accepted, the wife of that grandson becomes Mohan daughter-in-law, which means that Mohan is her father-in-law.

Step-by-Step Solution:
Step 1: Mohan mother is one generation above him, in the grandparent level of Mohan children.Step 2: The "grandson of my mother" is any male child of one of Mohan mother children. In particular, if Mohan has a son, that son is naturally a grandson of Mohan mother.Step 3: In exam questions, unless they explicitly force a cousin scenario, "grandson of my mother" is usually intended to be "my son".Step 4: Therefore, the grandson mentioned is taken to be Mohan son.Step 5: The girl is described as "the wife of the grandson". So she is married to Mohan son.Step 6: A woman who is married to a man son is his daughter-in-law, and the man is her father-in-law.Step 7: Thus Mohan is related to the girl as her father-in-law.

Verification / Alternative check:
Take an example with names. Let Mohan mother be Shanti. Shanti has a son Mohan. Mohan has a son Raj. Raj is a grandson of Shanti. If Raj marries a girl named Anu, then Anu is the wife of the grandson of Mohan mother. From Mohan point of view, Anu is his daughter-in-law. That directly matches the relationship father-in-law.

Why Other Options Are Wrong:
Mohan is not the girl husband because the girl is married to Mohan son, not to Mohan himself. He is not her direct father, because that would make the grandson his own son and also her brother, which contradicts the description of her as the wife. He is not her grandfather, because there is no extra generation gap implied between Mohan and his son wife.

Common Pitfalls:
Some learners confuse "grandson of my mother" with "my grandson", which would put Mohan one generation higher and might lead to the wrong idea that he is the girl grandfather. However, if the grandson is Mohan grandson, then his parent is Mohan child, not Mohan himself, and the age levels stop matching the intended exam pattern. The simplest and most standard reading is that the grandson of Mohan mother is Mohan son.

Final Answer:
Mohan is the girl father-in-law.