Let $A$ and $B$ be real symmetric matrices of same size. Which one of the following options is correct?

For the differential equation given below, which one of the following options is correct?

$$ \frac{\partial^2 u}{\partial x^2}+\frac{\partial^2 u}{\partial y^2}=0,0 \leq x \leq 1,0 \leq y \leq 1 $$

The divergence of the curl of a vector field is

If two unbiased coins are tossed, then what is the probability of having at least one head?