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Sum in ripple carry adder is --> $max( Sum, Carry)$ $for$ $MSB+$ $delay$ $to$ $produce$ $carry$ $of$ $N-1bits$

now given sum takes $34ns$ it can be clearly seen that for the last bit $Max(sum,carry)$ is $sum$ only

So the carry generation for $N-1=8-1=7bits$ has taken $90-34=56ns$

so time to produce carry $\dfrac{56}{7}=8ns$

$D$ $is$ $the$ $answer$

now given sum takes $34ns$ it can be clearly seen that for the last bit $Max(sum,carry)$ is $sum$ only

So the carry generation for $N-1=8-1=7bits$ has taken $90-34=56ns$

so time to produce carry $\dfrac{56}{7}=8ns$

$D$ $is$ $the$ $answer$

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