Answer
Reactions B and D conserve lepton number.
Work Step by Step
The lepton numbers for particles are in Table 44.2.
a. List the lepton numbers for the reactant(s).
$L_{\mu}$: +1
$L_{e}$: 0
List the lepton numbers for the product(s).
$L_{\mu}$: -1
$L_{e}$: 2
Not conserved.
b. List the lepton numbers for the reactant(s).
$L_{\mu}$: 0
$L_{e}$: 0
$L_{\tau}$: 1
List the lepton numbers for the product(s).
$L_{\mu}$: 0
$L_{e}$: 0
$L_{\tau}$: 1
Conserved.
c. We start with no leptons and end with one, so lepton number is not conserved.
d. List the lepton numbers for the reactant(s).
$L_{\mu}$: 0
$L_{e}$: 0
$L_{\tau}$: 0
List the lepton numbers for the product(s).
$L_{\mu}$: 0
$L_{e}$: 0
$L_{\tau}$: 0
Conserved.
