Question: By what process does thorium 230 decay to radium 226

 Thorium can decay to radium through alpha decay, alpha decay is the emission of an alpha particle, an alpha particle is helium nucleus,  it has an atomic mass of four and atomic number of two, likewise emitting an alpha particle by thorium will lead to loss of mass 4 from the subsequent by the thorium which is radium.

Th²³⁰ = ⁴he + ²²⁶ra