It's understandable that as a busy parent, you don't keep offering the same food if your child rejects it - after all, you want them to eat something and there aren't enough hours in the day to prepare food that just ends up on the floor or in the bin.
If your child is not otherwise fussy, it might not be worth offering a certain food many times - children often reject foods which have caused them stomach upsets in the past, for example, and this might be due to an intolerance or allergy.
If your child rejects many foods or even entire food groups though, it might be a case of them not having learnt to eat that food yet. Think of it this way; when your child started walking, they stumbled and fell many times but you didn't assume they would never walk. Research shows that it can take between 8 and 15 exposures for new foods to be accepted, but parents usually only offer a food 3 to 5 times before deciding their child doesn't like it.
This is easier said than done of course, but one way of encouraging your child to accept a food is to present it with familiar foods. If you want your child to learn to eat carrots, talk positively about them first. You could compare them to something else they like, or talk about how rabbits like them or point them out at the supermarket. The next time, you could offer them with a familiar and liked food, such as pasta, and just ask them to hold the carrot so they get used to it. Next time, you might encourage them to smell it and then next time, to try just a bite.
Try to avoid insisting or displaying anger as negative associations can backfire in learning situations - it might be helpful for you to think about a time when you were learning something new and how difficult it seemed at the start. Hopefully soon, your child will be well on the way to learning to eat (fill in the blank) food like a champ!