INDEX function can return a whole column, e.g. assuming you want to search for "x" in A1:Z1 try
=INDEX(A:Z;0;MATCH("x";A1:Z1;0))
Note: this doesn't return a text string like F:F, it returns a reference to the column in question which you would normally use within a function that expects a whole column reference, e.g. SUM, SUMIF, LOOKUP etc.
Edited: You can use the above in COUNTIFS function like this:
=COUNTIFS(INDEX(Source!A:Z;0;MATCH("x";Source!A1:Z1;0));Result!C3;Source!AT:AT;Result!$D$2)
That makes the first range dynamic (between columns A and Z - extend as required) based on where "x" is first found in Source!A1:Z1, e.g. if "x" is first found in J1 then COUNTIFS uses Source!J:J for the first range - you can do that for any of the ranges in COUNTIFS