The best answer is B. VPN.

A VPN (Virtual Private Network) is the standard solution for securely connecting two separate organizations or networks across an untrusted network, such as the internet. A site-to-site VPN provides:

secure connectivity between both companies

encrypted traffic in transit

protection of data confidentiality and integrity

a practical way to interconnect business networks

Why the other options are incorrect:

A. UTMA unified threat management device offers multiple security features, but it is not specifically the best answer for secure connectivity between two companies.

C. NACNetwork Access Control regulates which devices can connect to a network, but it does not provide intercompany network connectivity.

D. NGFWA next-generation firewall improves traffic inspection and filtering, but it is not by itself the main solution for secure network connectivity between two companies.

From the SY0-701 perspective, when secure communication between separate organizations is required, VPN is the most appropriate infrastructure solution.