A production function exhibits constant returns to scale when you:

$($a$)$ hold inputs constant$-$you double the output.

$($b$)$ double each input$-$you more than double the output.

$($c$)$ double each input$-$you less than double the output.

$($d$)$ double one input$-$you double the output.

$($e$)$ double each input$-$ you double the output