We know, Speed always inversely proportional to time $S\rightarrow \frac{1}{t}$
Here it is given $100sec.$ execution time.
Among which $80 sec$ for Multiplication operation and $20sec.$ for other operation
So, to improve the execution time, we must need to improve Multiplication time.
Other operation time will not change, because these are constant time taken by some hardware operation .
Now , to make it $4$ time faster, time will decrease when speed increase.
So, time will be $\frac{1}{4}$ th of original time, i.e. $25 sec.$
Therefore, time for Multiplication operation $=\left ( 25-20 \right )=5sec$
So, multiplication speed increased $=\frac{80}{5}=16$ times
Hence ans $3)$