The UNOPS Sustainable Investments in Infrastructure and Innovation (S3i) initiative focuses on three main sectors for its investments:

1. Affordable Housing: UNOPS aims to address the global housing crisis, particularly in developing countries, by financing affordable, high-quality and energy-efficient homes.

2. Renewable Energy: The initiative focuses on investments in renewable energy solutions, particularly in regions where energy access is a major challenge.

3. Health care: UNOPS is also active in the healthcare sector, investing resources in areas considerable requirements for improved health infrastructure.

You might notice that Mass Rapid Transport is not included in the answer. That`s because the UNOPS S3i initiative, currently, does not expressly mention mass rapid transport as one of its main focus areas for investment. So, this option “Only three” is the correct answer, as UNOPS S3i focuses on three of the given options, but not all four.