Based on your description and the context provided regarding oceanic crust around a mid-ocean ridge:
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The observation that can be made about the crust farthest from the ridge (yellow with dotted lines) is that it is older compared to the crust that is closer to the ridge. As new crust is formed at the mid-ocean ridge, the older crust moves further away from the ridge as new material is added.
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The process this observation proves is happening around a mid-ocean ridge is:
b The further from the ridge, the older the crust is: this proves that sea floor spreading is occurring and new oceanic crust is being made constantly.
This option highlights the continuous creation of new oceanic crust at the mid-ocean ridge and the movement of older crust away from the ridge, which is a key aspect of the theory of plate tectonics.