Correct Answer: Isomer

Solution : Given:
employ, oyster, error, ornate, tennis,?

Here, the last two letters of a word are the first two letters of the next word in the series.
empl oy → oy st er → er r or → or na te → te nn is
So, the next word will start with 'is'.
Let's check the options –
First option: Neptune→The first two letters are not "i" and "s".
Second option: nature→The first two letters are not "i" and "s".
Third option: terminate→The first two letters are not "i" and "s".
Fourth option: isomer→The first two letters are "i" and "s".

"Isomer" is the only one that follows the pattern. Hence, the fourth option is correct.