A convex lens is dipped in a liquid whose refractive index is equal to the refractive index of the lens. Then its focal length will

Lens maker's formula for a lens in a medium: $\dfrac{1}{f} = (\mu - 1)\left(\dfrac{1}{R_1} - \dfrac{1}{R_2}\right)$, where $\mu = \dfrac{\mu_{lens}}{\mu_{liquid}}$ is the relative refractive index.
Here $\mu_{lens} = \mu_{liquid}$, so $\mu = 1$.
$\dfrac{1}{f} = (1 - 1)\left(\dfrac{1}{R_1} - \dfrac{1}{R_2}\right) = 0$
$\therefore f = \infty$
Light passes through the lens without any bending (the lens becomes practically invisible), so its focal length becomes infinite.