(b)
$d(x,y) = max\{|x|,\ |y|\}$ …(i)
$but\ d(x,y) = a$ …(ii)
From, Eqs. (i) and (ii),
$a = {max\ }\left\{ |x|,\ |y| \right\}$
If $|x| > |y|,\$ then $a = |x|$
$\therefore\ x = \pm a$
$and\ if\ |y| > |x|,\$ then $a = |y|$
$\therefore y = \pm a$
Therefore, locus represents a straight line