'mostly opted' is grammatically correct and can be used in written English.
Example: After much debate, the group mostly opted for the more conservative option.
Following Augustine's dictum, Congress has mostly opted for future austerity.
Big companies in Detroit have mostly opted to part with their hometown.
I mostly opted for his choices, hoping they might make me look like a tall Asian man in his 20s.
The Rehnquist Court has mostly opted for color blindness, and Brown itself does not expressly prohibit that choice.
While roughly two dozen Packers wore short sleeves underneath their pads, the Giants mostly opted to bundle up with long sleeves.
The next meeting, a few weeks later, was for juniors and their parents — a chance for the guidance team to introduce an auditorium filled with fairly uninterested 16-year-olds (who mostly opted to sit with their friends) and their wide-eyed parents to the coming year's college preparation schedule.
Rather than high literature, he mostly opts for demotic texts, journalism and hack novels, to get a feel for the splits and bumps that marked private and public thinking.
In simple language, instead of calculating how to best achieve what they want in splendid isolation, people in networks mostly opt to follow the crowd a choice that is rational in its way, based on the assumption that in aggregate the crowd is better informed than the individual.
High risk families mostly opt for prenatal diagnosis.
"All types of properties are available, however foreigners mostly opt to buy condos or rent single-family houses, if their budget allows," Wagner said.
These events push us past the limits of our minds, and when given the choice of confusion or dismissal, we mostly opt for the latter.
