I don't know if you're saying you want to delete the data in specific columns for specific rows or if you want to delete specific rows all together. If it is the latter, there is another answer already that uses Delete Rows that would likely work for you. If you are trying to just delete specific data from specific records though, the typical workflow for a situation like this, at least to me, would be something like the following:
Make Feature Layer (No SQL expression at this point)
Select Layer By Attribute (write the selection for the first column here, ex: ColumnX = 'c' or ColumnX IN ('a', 'd') )
Calculate Field (set expression type to Python or Python_9.3, set the expression to 0 if a numeric field or '' if a text field, and select the column in question ex: ColumnX as the Field Name)
Select Layer by Attribute (make sure Selection Type param is set to NEW_SELECTION and then write a selection for the next field you want to delete data from, ex: ColumnY = 3 or ColumnY IN (1, 4) )
Calculate Field (use same parameters as before but do for the relevant column, ex: ColumnY)
Repeat this select Layer by Attribute and Calculate Field operation until you have all data cleared from all fields as desired. At the end, depending on if your workflow needs to continue onto other steps or not, you may want to run Select Layer by Attribute a final time and set the Selection Type parameter to CLEAR_SELECTION
Yes, this is a lot longer than you were probably hoping for, but... Again, this is if your question was getting at trying to just delete all the occurrences of c from column x in your example. If you were trying to delete the entire row/record for any records that have c as the column x value, then try Paul or Erica's suggestions.