What is the product of alpha decay of curium-226?
The question is flawed (possibly a misprint), your reasoning is sound (given the information provided to you).
Curium-226 does not exist, according to the Web Elements page on the isotopes of curium, the 'smallest' isotope is curium-240, which undergoes $\alpha$ decay to become plutonium-236.
$$\ce{^{240}_{96}Cm -&gt; ^{236}_{94}Pu + ^{4}_{2}\alpha}$$
The most likely candidate of what the question should have been asking about is radium-226 decaying to radon-222,
$$\ce{^{226}_{88}Ra -&gt; ^{222}_{86}Rn + ^{4}_{2}\alpha}$$
which is part of the uranium-series decay chain:

This is the most likely scenario, as the smallest isotope of thorium is thorium-227 (WebElements) and for radium, it is radium-223 (WebElements), both have too high a mass for anything with an atomic mass of 226 to directly decay into by alpha decay (as per the question).