Make records consistent ;

If you just want to complete the field name, you can use the following code:

let
    Source =  { 
        [ A = 1, C = 2 , D = 3] ,
        [ C = 2, A = 1, E = 3] ,
        [C = 4, E = 5]
    }, 
    result = List.Transform(
        Source, 
        each [A = null, B = null, C = null, D = null, E = null] & _
    )
in
    result

If you want to convert the records into a table and complete all the columns, you can use the following code:

let
    Source =  { 
        [ A = 1, C = 2 , D = 3] ,
        [ C = 2, A = 1, E = 3] ,
        [C = 4, E = 5]
    }, 
    result = Table.FromRecords(
        Source, 
        type table [A = nullable text, B = nullable text, C = nullable text, D = nullable text, E = nullable text], 
        MissingField.UseNull
    )
in
    result