The correct answer is: Although, precarious.
The word “although” is used to introduce a clause that expresses a contrast or concession. In this case, the contrast is between the broker’s warning that the stock was a precarious investment and the man’s insistence on buying a thousand shares. The word “precarious” means “uncertain or risky.”
The other options are incorrect because they do not express a contrast or concession. The word “because” is used to introduce a clause that expresses a reason or cause. The word “since” is also used to introduce a clause that expresses a reason or cause, but it is more formal than “because.” The word “as” is used to introduce a clause that expresses a purpose or function. The word “vulnerable” means “open to attack or harm.”
Here is a possible rephrasing of the sentence: “Although the broker had warned him that the stock was a risky investment, he insisted on buying a thousand shares.”