Final answer:
An investment trust fund is used when there are governments participating other than the government sponsoring the fund, the funds participating are part of the government sponsoring the fund, and bond covenants require that sinking funds be established by the sponsoring government.
Step-by-step explanation:
An investment trust fund is used when all of the above conditions are met. This means that government participation is required in the fund, including governments other than the one sponsoring the fund (option A), funds from the sponsoring government are involved (option B), and bond covenants mandate the establishment of sinking funds by the sponsoring government (option C).