Step 1: Define the problem in plain words.
A time averaged fossil bed contains remains that were not alive at the same time, even though they sit in the same layer.
Step 2: Ask which processes physically mix ages.
Burrowing animals churn the top part of the sediment column, dragging shells from lower, older layers up into younger ones. That is bioturbation, and it clearly mixes ages, so option A counts.
Step 3: Consider reworking by erosion.
When a current or wave cuts into older beds, it frees up old fossils and carries them into a newer depositional setting. These old shells then sit next to shells from the present time, so option B, erosion and redeposition, also mixes ages.
Step 4: Test compaction and pellet formation.
Compaction is just about squeezing sediment together as burial pressure increases, it does not transport fossils between time intervals. Pellet formation is about how fecal material shapes the sediment grains, again with no age mixing involved. Neither option C nor D causes time averaging.
Step 5: Conclude.
Only bioturbation and erosion with redeposition genuinely mix fossils across time.
\[ \boxed{\text{(A) and (B) are correct}} \]