Does space in between have any significance in IUPAC nomenclature?
$\boxed{\textbf{d}}$ is correct. It is the standard IUPAC name.

However, "2,4,6-trimethyl cyclohexanone" will often be understood as 2,4,6-trimethylcyclohexanone, similar to how, in theory, neopentane, 2,2-methylpropane and 1,1,1,1-methylmethane are all equivalent.