The strongest intermolecular interactions between pentane (c5h12) molecules arise from

The intermolecular interactions found between pentane molecules are London-Dispersion Forces.

Explaination:

Step 1:
           Instantaneous Dipole: As the pentane molecule is non polar in nature and the electron density on each molecule is symmetrical. When this symmetrical molecule approaches second symmetrical molecule there disturbance is caused due to repulsion between electrons. Due to repulsion the electrons changes there position and for a small period of time an instantaneous dipole is produced on pentane molecule.

Step 2:
           Induced Dipole: The dipole produced in step one when approaches another symmetrical pentane molecule, the partial positive part (low electron density site) attracts the electron from symmetrical pentane and induces polarity in it.

In this way the process repeats and interactions are developed between the non polar pentane molecules. 

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