in meromitic lakes no. of turnovers of water per year

1. 1
2. 2
3. many
4. zero

The correct option is 4.
A meromictic lake has layers of water that do not intermix. In ordinary, "holomictic" lakes, at least once each year there is a physical mixing of the surface and the deep waters. This mixing can be driven by wind, which creates waves and turbulence at the lake's surface, but wind is only effective at times of the year when the lake's deep waters are not much colder or warmer than its surface waters.