a. You can either talk to the manager about your problem or writing a letter to the president of the company.
- here the correct form would be "or write a letter" - so the whole sentence is incorrect
b. Anyone who is going to work here will have to be athletic, an intelligent person, and have a good sense of humor.
- yes, this one is correct! it doesn't have any mistakes AND it's a parallel construction.
c. Most people enjoy ice cream and the plant grows. - this is not incorrect, but it's a bit weird. plus, it's not a parallel construction
d. The painter wore glasses, gloves, and boots. - this is correct, but it's not a parallel construction.
