The correct answer is option D, about every 3 to 7 years
Reason -
El Nino comes at a regular interval of two to seven years. The last El Nino occurred in the year 2016 and it is forecasted that this year in 2019, the El Nino will come but its effect would not be as strong as that of year 2016. Also the El Nino in year 2019 will arrive late which shall means that the winters in 2019 will be a little warmer than the last year’s winter i.e 2018.