Explanation:

Let’s break it down, statement by statement:

- Assistant manager is the manager’s grandson.

- There must be a big age gap between manager and assistant manager.

- Cashier is the stenographer’s son-in-law.

- The cashier must be male and married to the stenographer’s child.

- Mr A is a bachelor.

- He can’t be anyone’s son-in-law or have kids.

- Miss D is the teller’s step sister.

- Miss D and the teller can’t be the same person.

- Mr E is the manager’s neighbour.

- Different people.

- Mr B cannot have a grandson or son-in-law; he’s only 20.

- So Mr B can’t be the manager or the stenographer.

- He also can’t be the assistant manager (needs a grandfather in the scenario), or cashier (needs to be someone’s son-in-law).

Now the options:

- Option 1: Mr B

- Fits the clerk role perfectly. He’s too young for the family relationships but can be a clerk.

- Option 2: Miss D

- She’s a step sister, not enough data, but less likely than Mr B.

- Option 3: Miss F

- No evidence she is the clerk.

- Option 4: None of the above

- One of the above fits better.

Correct answer: Option 1, Mr B

Mr B is the only one who fits the facts for being the clerk. He’s too young for the other roles that require family ties or marriage, and nothing in the statements clash with him being the clerk.