The major causes of change in population size are

Step 1: Analyze the question The question asks for the major causes of change in population size. In other words, what makes the number of people in a place go up or down?

Step 2: Explain the concept Population change is described by the demographic balancing equation:

Population change = (Births − Deaths) + (Immigration − Emigration)

• Natural increase or decrease comes from births and deaths.
• Net migration comes from people moving in (immigration) and out (emigration).

So the three components are births, deaths and migration.

Step 3: Check each option

• A. births, marriage and deaths: Marriage does not directly change population size. It may influence the birth rate, but it does not add or remove people. Migration is missing, so this is wrong.
• B. births, accidents and deaths: Accidents are only one cause of death, so they are already counted under deaths. Migration is missing, so this is wrong.
• C. births, deaths and migration: This covers all three direct factors that change a population's size. This is correct.
• D. migration, fertility and marriage: Fertility is related to births, and marriage is not a direct cause. Deaths are missing, so this is wrong.

Step 4: Answer The correct option is C. births, deaths and migration.

Tip: Remember "B-D-M" (Births, Deaths, Migration). Births and immigration add people, while deaths and emigration remove people. If an option leaves out one of these three or adds a factor like marriage, it is probably a distractor.