I assume you mean number 43, that's the only one fully visible and which you have highlighted with two circles. Normally I encourage learners to try and solve things for themselves but it is a difficult question for a learner. I can see where you are likely struggling. The answer is (C) because they believed (were under the impression) that she was still on the job. But the phrase "still on the job" can could and does have different meanings that may influence the answer depending on whether the text book may be American or British English. With this in mind it will be interesting to see what other answers you get from others. Are they Native American or Native British or something else.

In order to make answer (D) correct you have to put suspicion in the past tense, to keep all tenses matched = "she had resigned" . To make suspicion become a past tense you have to modify it to become "suspicious" Helen's parents were suspicious that she was still on the job, but she had resigned. "WERE SUSPICIOUS" as one phrase were/was = in the past. "SHE HAD RESIGNED" = IN THE PAST had (past) + resigned (past) Now the tenses match in all of the sentence.