A

Step 1: **Understanding Emission Spectra**
When atoms or molecules in a gas phase are excited, they emit light at specific wavelengths, producing what is known as an emission spectrum.

Step 2: **Types of Spectra**
There are three primary types of spectra: line spectra, band spectra, and continuous spectra.
- **Line Spectrum**: Produced by individual atoms or ions, characterized by sharp spectral lines at specific wavelengths.
- **Band Spectrum**: Produced by molecules, showing broad bands due to vibrational transitions.
- **Continuous Spectrum**: Produced by solid, liquid, or densely packed gases, exhibiting all colors without distinct lines or bands.

Step 3: **Identifying the Spectrum**
Since the question refers to the emission spectrum of a gas, it is likely a line spectrum because gases emit light at specific wavelengths due to the electronic transitions of atoms. In contrast, band spectra typically represent molecular gases and continuous spectra do not apply here.

Therefore, the correct option is A: "Is a line spectrum."