Explanation:

Let’s break down your options:

- Cell membrane:

- Surrounds all cells, not just plants, fungi, and algae.

- Controls what goes in and out but isn’t the tough, outermost layer—doesn’t offer strong structural support.

- Plastids:

- Only found in plant and algal cells (like chloroplasts).

- Handle things like photosynthesis, not structure or protection.

- Cell wall:

- This is the answer.

- It's a sturdy layer outside the cell membrane in plants, algae, and fungi.

- Gives the cell its shape, protects against mechanical stress, and stops the cell from bursting when water moves in. That's the tensile strength and protection you mentioned.

- Vacuole:

- Big sac inside plant cells, not around them.

- Helps with storage and maintaining pressure inside the cell, but not the external protective barrier.

So, “cell wall” is spot on. That’s the strong outer shell you were looking for.