Both. You can use it as singular or plural.
Jack. A signpost is a non-living thing and non living things do not grow. Sam is an adult and he has stopped growing already. Jack is a child and will continue growing the next few years.
I think the correct answer from the choices listed above is option B. A gene is a tightly coiled strands of DNA. A gene is the basic unit of heredity and is made up of DNA. It acts as instructions to make molecules. Hope this answers the question.