Whats your opinion on what should happen you one's health bar when they level up in RPGs

For example if your hp were at 100/150 and then you leveled up:

Should hp remain the same eg.100/200?
Should hp replenish completely eg.200/200?
Should hp lost remain the same eg.150/200?
Should hp replenish in ratio eg.133/200?

Debating activate!