Solution:

Multiplying by $b$ and subtracting $a$ gives $\dfrac{14b^2}{9a}=bk-a,$ an integer. Since $\gcd(a,b)=1,$ it follows that $a\mid14.$ Multiplying instead by $9a$ and subtracting $14b$ gives $\dfrac{9a^2}{b}=9ak-14b,$ so $b\mid9.$

Thus $a\in\{1,2,7,14\}$ and $b\in\{1,3,9\}.$ Checking the coprime candidates, the expression is an integer only for $(a,b)=(1,3),(2,3),(7,3),(14,3).$

So there are $4$ such pairs.

Thus, the correct answer is A.