Chapter 7 - Section 7.7 - Basics of Probability - 7.7 Exercises - Page 689: 1

The sample space is $S=${$H$}

During coin flip experiments we use the letter $H$ for heads and letter $T$ for tails. The sample space is a set that consists of all the possible results of the experiment. Here, as both sides of the coin are heads, the only possible outcome is that the coin will be heads up. The sample space is $S=${$H$}