The reason why there are only three $(03)$ baselines is because: $($choose the option which completely explains the question$)$

a$.$

Scope, Cost, Schedule are considered the triple constraints and accordingly provide the key performance indicators to measure Project Performance

b$.$

Scope, Cost, Schedule are considered the triple constraints as part of the Iron Triangle

c$.$

Scope, Cost, Schedule are considered the baselines and triple constraints and as such accordingly provide the key performance indicators to measure the Project Performance

d$.$

Scope, Cost, Schedule are considered baselines and as such are used in the monitoring and control process group only