<ul class="navigation">
  <li><a href="#">Inicio</a></li>
  <li><a href="#">Acerca de</a></li>
  <li><a href="#">Productos</a></li>
  <li><a href="#">Contacto</a></li>
</ul>

.navigation { 
  display: flex; 
  flex-flow: row wrap; 
  justify-content: flex-end; 
  list-style: none; 
  margin: 0; 
  background: deepskyblue; 
}
.navigation a{ 
  text-decoration: none; 
  display: block; 
  padding: 1em;
  color: white;
}
.navigation a:hover {
  background: #1565C0;
}
@media all and (max-width: 800px) {
  .navigation {
    justify-content: space-around;
  }
}
@media all and (max-width: 600px) {
  .navigation {
    flex-flow: column wrap;
    padding: 0;
  }
  .navigation a { 
    text-align: center; 
    padding: 10px;
    border-top: 1px solid rgba(255, 255, 255,0.3); 
    border-bottom: 1px solid rgba(0, 0, 0, 0.1); 
  }
  .navigation li:last-of-type a {
    border-bottom: none;
  }
}