Explanation:

- Highways are vital for economic growth, encouraging trade and connectivity.

- The blank refers to how private entities engage with the highway sector.

- Option 1: Investment - The correct choice as it directly refers to investors putting money into projects, specifying their lack of interest due to long-term commitments.

- Option 2: Transaction - Refers to the action of conducting business but does not fit well with the context of long-term projects and investor interest.

- Option 3: Operation - Implies running or controlling, not the act of investing or financial commitment.

- Option 4: Contract - Indicates an agreement but does not capture the reluctance of financial involvement described in the context.