Here are the corrections for each of the sentences you provided:

31. **D** ("frequently" should be replaced with "often" or removed, as "have you forgotten to lock the back door frequently?" sounds awkward.)

32. **B** ("had gone off" should be changed to "went off" to maintain the correct tense.)

33. **A** ("is usually swimming" should be corrected to "usually swims" or "is usually swimming" could be kept if it referred to a different time frame, but the present tense "is jogging" suggests the need for consistency, so "is usually swimming" should change to "usually swims.")

34. **D** ("to sunbath" should be changed to "to sunbathe," as "sunbath" is incorrect.)

35. **B** ("begin" should be corrected to "began" to maintain the past tense consistent with "Since" at the beginning of the sentence; it should be "Since I began school.")

So, the underlined phrases needing correction are:
31. D
32. B
33. A
34. D
35. B