Saif takes 10 hrs.
So total time they both would take without talking $=\frac{10\times 9}{19}=\frac{90}{19}$ hrs.
But they take 5 hrs $=\frac{95}{19}$ hrs.

So time wasted is  $\frac{5}{19}$ hrs.

They work for 5 hrs and put 10 bricks less per hour, so they had to put in extra time for $5\times 10=50$ bricks.
Now, extra time taken= $\frac{5}{19}$  hrs.
so, in  $\frac{5}{19}$  hrs: 50 bricks
5 hrs: x
x=950 bricks.
But this includes the 50 bricks extra put in the time  $\frac{5}{19}$  hrs. So total number of bricks is 900.

 Aamir takes 9 hours,$\Rightarrow$  he does $11.11\%$ of the work every hour

Saif takes 10 hours $\Rightarrow$  he does 10% of the work every hour

Together they finish  $21.11\%$ of the work

Given that they take 5 hours to finish the work after chatting, which means they are actually finishing  $\frac{100}{5}=20\%$ of the work

The remaining $1.1\%$ accounts for the 10 bricks decrease due to chatting

Thus, $100\%$  corresponds to 900. Answer is option (b)