The correct answer is beach volleyball.
To lift an object, Yahaira will mostly need power in her legs. Since freestyle swimming and road cycling are heavily cardio activities, she will not gain a lot of muscle mass in her legs, which is what she needs. Flag football would also be mostly cardio, since she wouldn't be accruing muscle mass to tackle others.
The answer is beach volleyball, because in volleyball, the majority of the power has to come from your legs when you bump, set, or spike the ball. This will increase her muscle mass, which will help her in lifting heavy objects.