This question uses the generalised definition of the replacement of a variable by a term for the extended syntax of first$-$order logic, as given in Def. $5\mathrm{.}31.$

What is the result of replacing x

by a

in the formula $$xp$($f$($x$),$x$)$

$?$

a$.$

$$ap$($f$($a$),$a$)$

b$.$

p$($f$($a$),$a$)$

c$.$

$$xp$($f$($a$),$f$($a$))$

d$.$

$$xp$($f$($x$),$x$)$