Anaesthetic that has a smooth induction is –
This question is straight forward as amongst the given options only thiopental is used as inducing agent.
Iv. inducing agents are used for induction because of rapid and smooth induction and drugs used for this purpose are:

Thiopentone
Methohexitone sod
Propofol
Etomidate

Till now we were discussing about inducing property of iv inducing agents. Some of the inhalation anaesthetic agents also have smooth induction.
Induction by inhalation agents 
Unpleasant

Ether

Intermediate

Halothane
Enflurane

Smooth

Isoflurane
Desflurane
Sevoflurane
Nitrous oxide

But, you should keep in mind that smoothest induction is produced by iv inducing agents and therefore they are used as inducing agent.
Diethyl ether
Thiopental
N2O
Halothane