Which of the sentences in the selection uses incorrect subject/verb agreement?

a)1 [1] "of who—of what is you thinking? " asked adele of her companion, whose countenance she had been watching with a little amused attention, arrested by the absorbed expression which seemed to have seized and fixed every feature into a statuesque repose.
b)2 [2] "nothing," returned mrs. pontellier, with a start, adding at once: "how stupid! but it seems to me it is the reply we make instinctively to such a question
c)5 [5] "oh! never mind! " laughed madame ratignolle.
d)3 [3]let me see," she went on, throwing back her head and narrowing her fine eyes till they shone like two vivid points of light.