If $x$ is an integer for which $3 x+1$ is even, then $3 x$ is odd, since it is 1 less than an even integer. If $3 x$ is odd, then $x$ must be odd (since if $x$ is even, then $3 x$ would be even). If $x$ is odd, then $7 x$ is odd (odd times odd equals odd) and so $7 x+4$ is odd (odd plus even equals odd). Therefore, the one expression which must be odd is $7 x+4$.