If the ph value was 3.6, what is the [h+]?
2.5 x 10-4
4.0 x 103

The formula here is:
pH = -log[H+]
3.6 = -log[H+]
[H+] = 2.51 x 10⁻⁴ mol / Liter

The first option is correct.

The density of water is 1 g/cm cubed and in order for the pebble to sink its density has to be greater than the density of water and in order for the twig to float its density has to be less than the density of water