Mi loves playing sports, so his brother hates it.

• D. should be "so his brother hates it" → Correction: The sentence is grammatically correct. There is no error.

Farmers should plough their fields carefully; however, they will not have a bumper crop.

• B. should be "their fields carefully; however" → Correction: The sentence is grammatically correct. There is no error.

In his free time, my grandpa adores to catch fish and feeding pigs.

• D. should be "feeding pigs" → Correction: It should be "to feed pigs" → Corrected sentence: "In his free time, my grandpa adores to catch fish and to feed pigs."

She hates use Facebook Messenger to discuss schoolwork.

• A. should be "use" → Correction: It should be "using" → Corrected sentence: "She hates using Facebook Messenger to discuss schoolwork."

Collecting coins are not only exciting but also relaxing.

• A. should be "Collecting coins" → Correction: It should be "is" → Corrected sentence: "Collecting coins is not only exciting but also relaxing."

When you play online games, be carefully if you want to make friends with strangers.

• A. should be "be carefully" → Correction: It should be "be careful" → Corrected sentence: "When you play online games, be careful if you want to make friends with strangers."

People in rural areas live much simpler than those in cities.

• A. should be "much simpler" → Correction: It should be "much more simply" → Corrected sentence: "People in rural areas live much more simply than those in cities."

I am not crazy in doing DIY. I am never able to finish any DIY projects.

• A. should be "crazy in" → Correction: It should be "crazy about" → Corrected sentence: "I am not crazy about doing DIY. I am never able to finish any DIY projects."

Are you keen on train your pets?

• A. should be "train" → Correction: It should be "training" → Corrected sentence: "Are you keen on training your pets?"

Children spend too much time to play video games.

• D. should be "to play" → Correction: It should be "playing" → Corrected sentence: "Children spend too much time playing video games."