Because the first letter of word "old" is "o" and "o" is a vowel pronunciation letter and we use with any vowel pronunciation letter article "an". So,here an old city is right.

Beauty is evergreen.

Right

In

Aviation means the flying or operating of aircraft.

wah mera bhai h.

Right

I cannot talk to right now, I am in the hotel, I am going to eat food.

How is your class going on?

Where are you from?