Why is reusing the same password online risky?

Correct answer: If one service leaks the password several accounts may be affected

The governing concept is the chain effect of credential compromise. If the same password is used on several websites and one service suffers a breach or phishing attack, an attacker may try that password on the other accounts. Thus option B is correct: one leak can expose multiple accounts. Reuse does not automatically improve security, does not make the password impossible to remember, and has no direct effect on internet speed. Unique strong passwords and multi-factor authentication reduce the risk.