A attack keeps the target so busy responding to a stream of automated requests that legitimate users cannot get in

Legitimate users cannot get in due to Spear phishing attack

Spear phishing Attack

Explanation:

A spear-phishing attack typically employs a collection of zombie processes which keeps the target so occupied and busy in responding to a stream of automated requests generated by these zombies. Due to this fact, the legitimate users cannot access the target. It is different from phishing. It is a targeted form of phishing. Spear phishing is an extremely common form of attack on businesses. Since it  is very much targeted, spear phishing is the most dangerous type of phishing attack as compared to other types of phishing attacks.

it would be a. excessive network collisions or d. slow connectivity between devices those would be my answer.