In the sentence: "We would like to work in the soup kitchen more frequently next year", the degree of comparison of the underlined adverb is B. Comparative.
In English grammar, an adjective or adverb can be in a particular form that implicates a comparative relation. This relation can be of more or less, or greater or lesser.
Comparatives are characterized by the suffix -er ("This house is bigger than the other one") or distinguished by the word more or less ("This job is more difficult than the other one").