Which chemical is the reducing agent in the redox reaction below?

Option (a) is correct.

A reducing agent is the one which loses electrons to other substance.

Here, Zn has oxidation number 0 in the L.H.S of the equation, but on R.H.S its oxidation number is +2 i.e. it Zn has donated two of its electrons to .

Hence, Zn is the reducing agent here. 

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

While unproven, it uses facts to support part of a conclusion. it is still factual and has predictive power, however.