To which group of the periodic table does an element having electronic configuration [ Ar ] 3 d 5 4   s…

So according to the question we have to identify the element whose electronic configuration is give in the terms of noble gas

So in periodic table the $\mathrm{d}-$ block are known as transition element

So we know that 

$\mathrm{Mn} = \left[\mathrm{Ar}\right]3{\mathrm{d}}^{5}4{\mathrm{s}}^2$

So we know that from above electronic configuration that it is belong to the seventh group in the periodic table