Secular trend means -: (A) Seasonal changes, (B) Periodical changes, (C) Religion changes, (D) Long term changes
Answer is D. Long term or secular trends 

The term secular trend implies changes in the occurrence of disease over a long period of time, generally several years or decades.
There may be progressive increase or decrease in the occurrence of disease.

Examples -

CHD, lung cancer, and DM have shown a consistent upward trend in developed countries.
TB, typhoid fever, diphtheria and polio have shown a declined trend in developed countries.