The correct option is B Lady Bertram's husband
"For though Lady Bertram, rather at home in the epistolary line, having early in her marriage, from the want of other employment, and the circumstance of Sir Thomas's being in Parliament," - through this sentence we can infer that Sir Thomas is Lady Bertram's husband. The sentence explains that one of the reasons behind Lady Bertram got into the habit of writing letters and maintaining correspondence is her husband's, Sir Thomas's, work in the Parliament. Sir Thomas is definitely not Lady Bertram's son; he is also not a boarder at Mansfield or a relative of the Grants and he is not a friend of Lady Bertram's. Option B is the best answer.