There is a phase in compiler processing called as code optimization either it could be Machine independent or dependent .
So the term minimum here always refer to optimization and that too without losing the crisp of the program i.e correctness of program should be maintained.
here,
$c=b+a$
$e=c-a$ $\color{Red}{\text{(This is redundant)}} \ b+a-a=b$
$f=c \times e$
$h=c+a$
$i=h+f$
Therefore,
$T_1=b+a$
$e=c-a$ $\color{Red}{\text{(Redundant)}} $
$T_2=T_1 \times b$
$T_3=T_1+a$
$T_4=T3+T_2$