Convert categorical variable into binary columns in R [duplicate]

Question

I made the stupid mistake of enabling people to select multiple categories in a survey question.

Now the data column for this question looks something along the lines of this.

respondent	answer_openq
1	a
2	a,c
3	b
4	a,d

using the following line in r,

datanum <- data %>% mutate(dummy=1) %>%
spread(key=answer_openq,value=dummy, fill=0)

I get the following:

However, I want the dataset to transform into this:

respondent	a	b	c	d
1	1	0	0	0
2	1	0	1	0
3	0	1	0	0
4	1	0	0	1

Any help is appreciated (my thesis depends on it). Thanks :)