For a person:
* Sérieux: This is the most common word and translates to "serious" in the sense of being responsible, reliable, and not frivolous.
For a situation or issue:
* Grave: This word is used for something serious in the sense of being important, dangerous, or requiring attention.
* Sérieux: This word can also be used for situations, especially when referring to a serious problem or issue.
Other options:
* Solennel: This word is used for something serious and formal, often with a religious or ceremonial context.
* Grave: This word can also be used for something serious and important, but it has a more somber tone than "sérieux."
The best word to use will depend on the specific context and nuance you want to convey.