Hi,
Kushchokhani
In C, 'could have been' indicates that action of buying stocks directly began in past and continues in the present.
could have been bought actually indicates that
the buying never happened. Many people could have bought securities themselves but they actually didn't. Instead, they turned to stockbrokers.
So,
could have verbed is used for the things that had a chance to happen but didn't. For example, you
could simply have googled and learn about it by yourslef, but most probably you didn't. Indeed, you have to google such simple grammar staff beacuse you won't have to have wait for the answer. Check
here.
Could be is used for current or future events that are not real.
I like Sara very much. But she is already married. I think we
could be the best couple = We are not the best couple
right now.I am very hungry but don’t have cash on me to buy hamburgers. I
could easily eat ten of them = I can’t eat ten of them because I don’t have money
right now.
Finally I am a millionaire and flying in my own jet to Monaco. You
could be here with me, but you are with another man = you are not with me
right now.Could have been is used for past events that are not real.
Yesterday I was very hungry but didn’t have cash on me to buy hamburgers. I
could have easily eaten ten of them = I didn’t eat ten of them because I didn’t have money yesterday (in the past).
Did Sara really come to see me yesterday when I was not at home? How pity. I
could have talked to her for hours = I didn’t talk to her for hours because I wasn’t at home yesterday (in the past)
Now back to the problem: Not trusting themselves to choose wisely among the wide array of investment opportunities available
in the 1990s,...You can see that this event took place in 1990s, that is in the past. So you should use
could have bought.
If sentence talks about current events, then you can use
could be: People can’t choose and turn to brokers even today. They
could buy themselves.
Also read about subjunctive Mood:
https://grammarist.com/grammar/subjunctive-mood/