At the end of the year $1999, 129$ Naya products are still in operation(acc. to the line graph). This includes in-operation products of the year $1998$ as well as $1997.$
So, effectively we subtract operational products from both $1997+1998$ from $129$ to get new products bought in the year.
$20 \%$ of $30$ (products bought in $1997$) is disposed off in $1999$ or $6$ products are disposed off, so $24$ products from $1997$ are still in operation.
$50$ new products were bought in $998(80-30).$ Total operational products from $1997 \& 1999=24+50=74.$
Therefore, new products purchased in $1999=124-74=50$
Option $\text{(B)}$