Which compound has the lowest pka? (ch3ch2o2c) 2ch2 ch3ch2o2cch2cho ch3ch2coch2cho phcoch2coch3 (nc) 2ch2?

Aldehydes having the lowest pKa value around 17 and thus the other carbonyl have higher pKa than aldehyde.

Now, two aldehyde are present in given sereis such as

ch3ch2o2cch2cho and ch3ch2coch2cho

ch3ch2coch2cho is more acidic or having low pKa than ch3ch2o2cch2cho due to the presence of carbonyl at alpha position of aldehyde and rasonance will take place in ch3ch2coch2cho which result in release of proton readilly.

Hence, ch3ch2coch2cho having low pKa value of all and thus is highly acidic of all.