Which ones are weak and which are strong?

Strong bases are bases that virtually completely dissociate in an aqueous solution.
Weak bases are bases that only somewhat dissociate.

• #Sr(OH)_2#

We can see that the base is #Sr(OH)_2#, and that it basically completely dissociated when placed into an aqueous solution.
So, we can safely say that this is a strong base.

• #NH_3#

Our original base is #NH_3#, and we can see that, while some dissociated in water to form #OH^-# and #NH_4^+# ions, many still stay as #NH_3#.

#NH_3(aq) + H_2O(l) rightleftharpoons NH_4^+(aq) + OH^(-)(aq)#

This means that it does not completely dissociate in water, therefore making it a weak base.

• #NaOH#

#NaOH# dissociates basically completely in an aqueous solution to form #Na^+# and #OH^-# ions, just like #Sr(OH)_2#, so it's a strong base.

• #NaHCO_3#

This image is a bit more difficult to understand, because there are actually two reactions taking place here.

The first reaction is the dissociation of #NaHCO_3# into #Na^+# ions and #HCO_3^-# ions.

#NaHCO_3(aq) -> Na^+(aq) + HCO_3^(-)(aq)#

This happens with basically every #NaHCO_3# compound, but that doesn't make #NaHCO_3# a strong base because, while it virtually completely dissociates, it's not a base.

Then, some #HCO_3^-# ions accept a proton from #H_2O# molecules to form #H_2CO_3# and #OH^-#:

#HCO_3^(-)(aq) + H_2O(l) rightleftharpoons H_2CO_3(aq) + OH^(-)(aq)#

But we can see that this reaction doesn't happen too often, because many are still staying as #HCO_3^-#.
Therefore, this would be a weak base.