The question deals with isotopes and how their masses differ from their normal elemental counterparts.
Here, T represents tritium, which is an isotope of hydrogen with a mass of 3 amu. Similarly, the isotope of oxygen present is ¹⁷O, which means it has a mass of 17 amu.
To determine the mass of this molecule, we do the following calculation:
3 * 2 + 17 * 2
6 + 34
40
The mass of T₂¹⁷O₂ is 40 amu