A Gallery of Groups · Easy problem

Yes, $21$ is a unit mod $35$ (since $21$ is odd)

Yes, $21$ is a unit mod $35$ (since $21 < 35$)

No, $21$ is not a unit mod $35$ (since $\gcd(21,35)=7$)

No, $21$ is not a unit mod $35$ (since $21$ is even)