Mustard flowers

Seema, a great reference for plants in your area is http://www.calflora.org/ Be sure to select your county before searching. In this case, you can search both Descurainia and Brassica in your area and it will show you the known species and even var. This will help you narrow it down.

Common mustard's seed pods (siliques) do not point so vertical from what I've seen. They stick out more perpendicular to the stem. The Descurainia species that I know all have mostly vertical pointed siliques. I could be wrong about this.

Seema, do you have other photos of this plant? The base of the plant should have a different shape of leaf which would help with the ID.