The correct answer is B) Use an explosimeter. An explosimeter, also known as a combustible gas indicator or CGI, is the generally accepted method for determining the atmosphere within a cargo tank. Explosimeters are designed to measure the concentration of flammable gases or vapors in the atmosphere, allowing the operator to quickly and accurately assess whether the atmosphere is in the explosive range. This is in accordance with regulations and industry best practices for safely evaluating cargo tank atmospheres. The other options are not the preferred methods. A) Using an open flame test is unsafe as it could ignite the atmosphere. C) Entering the tank with an oxygen analyzer does not directly measure the flammable gas concentration. D) Sending a sample for laboratory analysis takes time and does not provide the immediate information needed to ensure safety.