I am a Geography teacher at school, I have a problem with the children as they always question me "what proves that the earth/world is rounded?". is there anyone who can help me answer this question clearly?

You can explain to your students with an example:

Explanation:

If you’ve been next to a port lately, or just strolled down a beach and stared off vacantly into the horizon, you might have noticed a very interesting phenomenon: Approaching ships do not just “appear” out of the horizon (like they should have if the world was flat), but rather seem to emerge from beneath the sea.

But—you say—ships do not submerge and rise up again as they approach our view (except in Pirates of the Caribbean, but we are hereby assuming that was a fictitious movie series). The reason ships appear as if they "emerge from the waves" is because the world is not flat: It's round.

If you stick a stick in the (sticky) ground, it will produce a shadow. The shadow moves as time passes (which is the principle for ancient Shadow Clocks). If the world had been flat, then two sticks in different locations would produce the same shadow:

Imagine the Sun's rays (represented by yellow lines) hitting two sticks (white lines) some distance apart. If the Earth were flat, the resulting shadows would be the same length, no matter how far apart you place the sticks.

         But they don’t. This is because the Earth is round, and not flat

Sticks placed at distant locations will throw shadows of different lengths.

answer; (d) melting of sea ice;