a$)$ The register used to store the equivalent of the for loop's i variable is $t$0.$

b$)$ The assembly language instructions that correspond to i $<100$ are addi $t$1,$ $zero, $100$ and bne $t$1,$ $t$0,$ LOOP.

c$)$ The assembly language instruction that corresponds to i$++$ is addi $t$0,$ $t$0,1.$

d$)$ The assembly language instructions that correspond to sum $+=$ A$[$i$]$ are lw $t$3,0($$s$0)$ to load the value of A$[$i$]$ into $t$3,$ add $t$4,$ $t$0,$ $t$3$ to add the value in $t$3$ to sum in $t$0,$ and addi $s$0,$ $s$0,4$ to increment the base address $s$0$ to point to the next element in the array.