Choose the word OPPOSITE in meaning to the underlined word.
Option C is the right answer.
sporadic(adj): occurring at irregular intervals | Source: https://dictionary.cambridge.org/dictionary/english/sporadic
The questions asks for the OPPOSITE of ‘sporadic’, which is an adjective (adj).
Therefore, Option C Frequent(adj) is the right answer.
Option D Regularity(n): quality of being regular is NOT applicable here.