Here are the corrections for each sentence:

1. **A** - "At weekends we often go swimming and **spend** time at the park if it is fine." (Change "spending" to "spend")

2. **C** - "Farmers should plough their fields carefully; otherwise, they will not have a bumper crop." (Change "careful" to "carefully")

3. **A** - "In his free time, my grandpa enjoys **catching** fish and feeding pigs." (Change "to catch" to "catching")

4. **B** - "They have built a **new** tunnel through the mountain." (Change "newly" to "new")

5. **A** - "Can I have a few **cans** of condensed milk in the coffee, please?" (Change "few" to "few cans of" or "some" depending on context)

So the options that need correcting are:

1. A
2. C
3. A
4. B
5. A