a) .ogv
b) .aac
c) .wav
d) .webm

Answer: a
Explanation: For accessing audios in different browsers we can use various extensions like .aac, .mp1, .mp1, .m4a, .mp2, .mpg, .mp3, .mpeg, .oga, .ogg, .wav, .webm. For video we have file extensions like .mp4, .ogv, .m4v, .webm. It is noticeable that some extensions are common both in audio and video. So user should pay attention where to put these as per the need.