Who decides on the financing system, interest crediting and the regulations? The answer is neither the Federal Council nor the employer alone.
The supreme body of a pension institution is the board of trustees. It stands at the top and bears overall responsibility for the pension fund's affairs. A particular feature of the Swiss system: the board of trustees is composed on a parity basis. This means that employer and employee representatives sit at the table in equal numbers and decide together. This parity ensures that no single side decides alone over the insured persons' money. The board of trustees sets the strategic guard rails, appoints the management and oversees that the pension fund is run in the interest of all those insured.
This makes clear why the other answers do not fit. The Federal Council regulates the legal framework but does not run any individual pension fund. The supreme body is and remains the board of trustees.