All life as we know it is carbon-based: carbon is an ideal element for building complex organic molecules, because it has the ability to form long chains of interconnecting covalent bonds.
There’s no concrete reason why carbon should be the only basis of life, though, so scientists haven’t ruled out other options. There’s actually a term called “carbon chauvinism”, coined by Carl Sagan, which refers to the assumption that we should only look for carbon-based extraterrestrial life—we just don’t know enough about the universe to assume something as limiting as that.
Some have proposed that life forms could be made instead of silicon, nitrogen, phosphorus, boron…but we’ve never come across any empirical evidence, so for now carbon remains the basis all known biological life.
This is good common science to know.
And yeah, silicon is probably the most theorized since it neighbors carbon.
Carbon is a slut.