Four Bells toll together at $9:00$ A.M .They toll after $7,8,11$ and $12$ seconds respectively . How many times will they toll together again in the next $3$ hours?

The correct option is C.$5$

To find the interval at which all the four bells toll together, we will find the LCM of $7,8,11,12$.

$\begin{array}{cc}2& 7,8,11,12\\ 2& 7,4,11,6\\ 2& 7,2,11,3\\ 3& 7,1,11,3\\ 7& 7,1,11,1\\ 11& 1,1,11,1\\ & 1,1,1,1\end{array}$

LCM = $2\times 2\times 2\times 3\times 7\times 11=1848$ seconds

Now,$1$ min =$60$ seconds

Therefore, $1848$ seconds = $\frac{1848}{60}=30.8$

Now, number of times it will toll together in $3$ hours ($180$ mins) = $\frac{180}{30.8}=5$ (approx.)