A flood of packets with invalid source-IP addresses requests a connection on the network. The server busily tries to respond, resulting in valid requests being ignored. What type of attack occurred?

SYN flooding

Explanation:

The options are:

Trojan horse brute force ping of death SYN flooding

This is certainly a denial of service attack. And the Syn flooding is a kind of such attack in which the attacker transmits a series of SYN requests to the target for consuming enough server resources cuch that the system becomes unresponsive to the legitimate traffic. And hence, the correct option out here is the SYN flooding.