Explanation for other options:

☀ The Officials were worrying about that severe natural calamity in the northern region. This option changes the sentence to the active voice and alters the verb tense. It suggests that the Officials were worrying about the severe natural calamity in the past, which is not the intended meaning of the original sentence.

☀ The Officials are worrying about the severe natural calamity in the northern region. This option changes the sentence to the active voice but maintains the original meaning. It states that the Officials are currently worrying about the severe natural calamity in the northern region.

☀ The Officials were worried about the severe natural calamity in a northern region. This option changes the sentence to the active voice but alters the preposition "in a northern region" instead of "in the northern region," which changes the meaning slightly and does not match the original sentence.

☀ The Officials are worried about the severe natural calamities in the northern region. This option changes the sentence to the active voice but pluralizes "calamity" to "calamities," which changes the meaning and does not match the original sentence. The original sentence talks about a specific severe natural calamity, not multiple calamities.