Genetic characteristics that we can't control include
a.
height and amount of muscle fibers
b.
amount of muscle fibers and heart rate
c.
heart rate and flexibility
d.
flexibility and amount of daily exercise

I believe the answer is A. The term genetic characteristics can allude to a genotype, which is the particular DNA grouping that codes for a characteristic, or a phenotype, which is the means by which the particular DNA succession shows itself in the living being regularly, however not generally, the phenotype is the manner by which the creature looks.

Genetic characteristics that we can't control include A. height and amount of muscle fibers. There is nothing we can do about the height - it really depends on our parents. If both of them are tall, the chances are that we are going to be tall as well, and vice versa. This is something that genetic modifications cannot affect yet, as well as amount of muscle fibers, although I believe in the future that too will be possible.