What was Einstein's greatest mistake?

D - none of the above

He made those lousy Einstein bagels. Big mistake!

k)

You will probably find he accepted the universe 'could be' expanding.
Possibly whatever you read was published in terms of receptive value.
IE: the populous must hear confidence.

As evidenced in E=MC2 and its flaw, relative to stated values.
Without a general theory of unified simplicity, quite so many cannot extrapolate the accuracy of further discoveries from a collective area of worth.

Also, if the universe to you is more than merely space time alone, it may not only be expanding, but halting and possibly slightly receding in non verifiable quantities. To which extents varies on view point, perhaps only at the outskirts.

e.

And there are some who argue that he apparently was wrong about being wrong on that.