Concept:
The days of the week repeat after every 7 days.
Therefore we divide the given number of days by 7 and use the remainder.
Step 1: Find the remainder when 126 is divided by 7.
\[
126 \div 7 = 18
\]
\[
\text{Remainder}=0
\]
Step 2: Interpret the remainder.
A remainder of 0 means an exact number of complete weeks.
Hence after 126 days the day remains the same.
\[
\text{Sunday}
\]
Step 3: Check the given options.
The mathematically correct answer is
\[
\boxed{\text{Sunday}}
\]
However, since Sunday is not among the options, the official key for this question appears to have a printing error.