In an experiment with a simple pendulum a student recorded 10 complete oscillations in 25 seconds. The effective length of the pendulum is a)10 b)25 c)2.5 d)0.4

So what have you determined? We will help but not do your homework. How do you determine pendulum length?

Length:http://easycalculation.com/physics/classical-physics/images/simple-pendulum-length.gifT= period, L = length, g= acceleration of gravity

None of the above.