You can set here the solver type to one of diagonal, matrix_iterative_bicg, matrix_iterative_petsc or matrix_superlu. The solver set here holds for the entire calculation (as opposed to the control_options_solver which only holds for the corresponding time steps). In fact, each control_options_solver will be overwritten by a specified options_solver.
You can also set an environment symbol TOCHNOG_OPTIONS_SOLVER to solver, by example to -matrix_superlu. In this way, you do not need to set the solver type in each input file separately.