The correct answer is b. the overload principle. The overload principle says that muscle strength is only gained if you "overload" and work your muscles harder than they are used to working to.
A is not correct because the specificity principle solely means that you should condition yourself for a particular activity. Lifting weights doesn't mean you're planning to be a pro weight-lifter; therefore, you can cross that one out.
C is not correct because reversibility principle is the fact that muscles rapidly lose strength as soon as one stops exercising as much -- the progress is basically reversed. This is basically the opposite of increasing the workload during an exercise.
D. is not correct because increasing the amount of weight is not synonymous to increasing the intensity. You would have to take several factors into account, such as the number of reps.
Did this same question in my aerobic fitness class a month ago, so I hope this helps! :)
