First, examine at the atoms directly attached to the stereocenter of the compound. A substituent with a higher atomic number takes precedence over a substituent with a lower atomic number. Hydrogen is the lowest possible priority substituent, because it has the lowest atomic number . So cl is assigned highest priority and then methyl and ethyl grp are checked both have first carbon attached to stereospecic centre as Carbon so we compare second attached atom of methyl and ethyl which is Carbon in case of ethly and H in case of methyl thus Ethyl is 2nd highest priority and methyl as third .

Now This configuration is clockwise in direction and thus R 

But why not option c is the answer 

Because it is anticlockwise confirguration.