As a system administrator, you are responsible for maintaining the website of your company which deals in online recharge of mobile phone cards. One day to your surprise, you find the home page of your company’s website defaced. What is the reason for webpage defacement?
Denial of Service attack
Session Hijacking
DNS attack through cache poisoning
Buffer overflow

DNS Attack through cache poisoning means Domain Name System through cache poisoning.
DNS protocol is a system in which the user friendly domain name is converted into the computer friendly IP address.
In this DNS attack through cache poisoning, the attacker corrupts the DNS server which gives the fraudulent or malicious IP address instead of the correct IP address.
So, in this the website traffic is redirected to a fake website.
In this case, the user is of the opinion that he / she has entered the correct domain name but internally the IP address is corrupted and so the user is seeing a defaced website.

Option (a) Denial of Service Attack is different from this.This is not related to the case stated here. So, option (a) is wrong.

Option (b) Session Hijacking is a different attack from this. This is not related to the case stated here. So, option (b) is wrong.
Option (d) Buffer Overflow is a different attack from this. This is not related to the case stated here. So, option (d) is wrong.