Answer
b) is correct.
Work Step by Step
The impulse-momentum theorem states that $$J=mv_f-mv_0$$
As you jump straight down from the same height in both cases, your mass $m$, initial velocity, $v_0$, and final velocity $v_f$ are the same in both cases, too. So, the sand and the water exert the same impulse on you.
We also have $J=\overline F\Delta t$
Because the impulse is the same but $\Delta t$ for sand is smaller than $\Delta t$ for water, $\overline F$ for sand is greater than $\overline F$ for water. In other words, the sand exerts a greater average force on you.
b) is correct.
