Which compound has the highest covalent character?

Which of the following biomolecules acts as specific catalysts in biological reactions?

Which of the following oxides is the most basic oxide?

$AB$ crystallizes in a body-centred cubic lattice with edge length '$a$' equal to $387 \ pm$. The distance between two oppositely charged ions in the lattice is ............. $pm$.

If $[x]$ denotes the greatest integer not exceeding $x$ and if the function $f$ defined by $f(x)= \begin{cases} \frac{a+2 \cos x}{x^2} & , x < 0 \\ b \tan \frac{\pi}{[x+4]} & , x \geq 0 \end{cases}$ is continuous at $x=0$,then the ordered pair $(a, b)$ is equal to

$A$ marble block of mass $2\, kg$ lying on ice when given a velocity of $6\, m/s$ is stopped by friction in $10\, s$. Then the coefficient of friction is-