The past progressive tense is used to describe actions in the past that are progressive or continuing. Due to this, this tense is often used to introduce new actions in the past. Additionally, in terms of grammar, this tense includes the auxiliary verb "was" or "were" followed by an -ing verb form. For example "I was going home when she called me".
According to this, the sentence in past progressive tense is "She was surfing" considering this follows the structure of this tense as it contains the auxiliary verb "was" and the -ing verb form "surfing" that indicates continuing action in past.
